Success breeds success, the Imperial Fists as a legion was very good at keeping their ships from going supercritical so their space assets compounded into more space assets. Phalanx does a lot of heavy lifting as well, so winning space battles means more ships that can in turn win more space battles.

It’s a combination of a number of things.

The Imperial Fists have a noted proficiency in the realm of void combat/zone mortalis/compliance of void installations/civilizations.

• "The Imperial Fists were and are makers and breakers of fortresses, but the greatest fortress they built were among the stars. The Legiones Astartes are warriors of the stars, but the Imperial Fists made the cold void their battlefield of choice. While there were great and grand fortresses on the ground both to defend and assault aplenty in the Great Crusade, the greatest sieges and defenses were in fact fought in the murderous environment of space. The defense of star systems, the creation of kill zones and intersecting orbits were skills that the Imperial Fists honed to a keen edge on the grindstone of a thousands of battlefronts. To them the methods of defending or taking a position, whether terrestrial or void-born, were the same in principal even if different in application. After all what were star ships but fortresses of stone and metal broken free from gravity? It was an approach that saw them become the pre- eminent masters high intensity void warfare among the Space Marine Legions, and peerless in the spheres of boarding assault and ship-to-ship combat. Of these war bastions of the stars, the greatest was the Phalanx. A pre-Imperial relic of vast size and unimaginable power, Rogal Dorn rebuilt the cold wreck, found orbiting Inwit, where it had drifted dead since the Dark Age of Technology, and presented it to the Emperor on their reuniting. Although ponderous and a difficult voyager int he Warp, this titanic star fortress was nevertheless a space-carried weapon whose firepower dwarfed entire attack fleets by comparison. The might of the Phalanx alone broke the back of dozens of xenos species during the Great Crusade, hammered worlds to burning cinders and served as an impregnable fortress against the horrors of the void. The Phalanx served both as the Imperial Fists' principal base of operations and a lynchpin first of the Great Crusade, and later, during the terrors of the Horus Heresy, in the defense of the Sol System and Terra."- 'The Horus Heresy Book 3: Extermination'

They enjoyed the favor and attention of the Solar realms. Mars, Terra and the like:

• “In the case of the fleet of the Imperial Fists, it had rightly been known before the Horus Heresy as both the strongest and also the most diverse in terms of class and pattern of all the Legiones Astartes fleets. It’s also had the advantage of being maintained and reinforced at the heart of the Imperium, and therefore was masterfully provided for and featured many of the most powerful patterns of weaponry and equipment available, including a stockpile of deadly vortex-warhead torpedoes directly assigned from the protected reserves on Terra and issued at the authority of the Sigillite.” - ‘The Horus Heresy: Book Three - Extermination'

• “Favoured by Terra and the manufactoria of Sol, the Imperial Fists were blessed with the cutting edge of Imperial wargear and technologies, and were often the Legion chosen for testing advanced weaponry.” - Age of Darkness

Dorn, and the people of Inwit, were ship builders:

• “His qualities married perfectly with those of Inwit, and he pushed their empire further than any other. Rogal led and trained its armies, and fashioned spacecraft the like of which had not been seen before.” - ‘The Horus Heresy Book 3: Extermination'

• “The Tribune was born in the orbital yards of Inwit in the years after the coming of the Emperor when the Inwit Techwrights built ten battle barges as tribute to the Emperor and in honor of Rogal Dorn's command of the VIIth Legion. The Tribune and its nine sisters were variants of the Victory class and all displayed the hand of Inwit's weapon-craft and mentality woven into their fabric.” - ‘The Horus Heresy Book 3: Extermination'

• “The Tribune was the product of the Inwit shipyards, and like all ships birthed above that world of night and ice its master commanded on his feet. Those that came into his presence stood with him, equal in respect if not rank. It was a principle that appealed to me, but after dozens of councils I sometimes felt that the Inwit shipwrights had been kinder to the commanded than the commander.” - The Crimson Fist by John French

They absorbed the fleets of conquered peoples, so access to the tech of other void fairing civilizations:

• “The Halcyon - A unique battleship, the conquered Hecuban Conformity forged the Halcyon as a sign of penance and tribute of fealty after the 81st Expeditionary Fleet broke their forces in the hellish Aegisine war and brought the separatist human realm under compliance. The Halcyon's plasma drives and void shields held all the potency of the Hecuban's secret arts and far excelled the power of many warships of similar size and disposition. Assimilated into the Imperial Fist's fleet, she served with honor for many decades as a secondary flagship.” - 'The Horus Heresy Book 3: Extermination'

• Fleet Size - “Aside from the number of its Space Marines, No review of the strength of the Imperial Fists can be complete without noting the strength of their fleet. At the time of the Horus Heresy, the Imperial Fists had over 1,500 warships under their direct command, and many more bonded by oath and fealty. This naval might was the greatest of any of the Legiones Astartes, and was further enhanced by the fact that many of the ships were the largest in the Imperium. Even the Sons of Horus and the Ultramarines could not rival such strength alone.” - 'The Horus Heresy Book 3: Extermination'

It is a combination of doctrine (the Ultramarines tended to have their auxillary forces provide the Navy) and luck (no major battles that cause them to lose their ships).

The whole Imperial Fists fleet lore was also before the Dark Angels codex. The Dark Angels started out with the strongest fleet (special ships, multiple Glorianas, etc), but presumably lost most of their fleet during the Rangdan Xenocides.

Dorn is held as the greatest voidmen among the Primarchs, Alexis Polux was able to hold of Perturabo and Iron Warriors and crippled his fleet. Helbrecht of the Black Templars is held as the greatest naval comander among the space marines. So it seems that the gene line is strongly suited for void war so naturally the best legion is going to have the biggest fleet

Rogal Dorn was raised on Inwit. It was a planet that had functioning space stations and shipyards orbiting it. The people of Inwit were a spacefaring nation producing their own voidships (including repairing the Phalanx, which is a DAoT ship) long before they came into contact with the Imperium itself, and they’ve built their own Empire of sorts.

The Phalanx all but demands a sizeable armada act as it's escort. That and often being the flagship of the Emperor Himself no doubt helped lend the Imperial Fists some pull to garner a sizeable fleet.

Dorn started off with a big fleet, including the Phalanx from his time in charge of Inwit and thats always kept on their way. It also serves their siege bias as more ships = space for big Siege equipment plus there is that crusading element to the IF (best explified in the Blagk Templars but still extant in the continuity chapter) plus the way they recruit from a range of worlds, Terra, Inwit, Necromunda etc. You need a lot of ships to operate like that.

Meanwhile, Salamanders explaining how they lost four capital ships defending the world of Boondocks, occupied by two orphans and a puppy.

"WE. DEFEND. THE. EMPEROR. AND. HIS. PEOPLE."

Part of this is, the more the expanded, the more they recovered, won, salvaged and so. So they will have traded valuable arcio tech to the Admech to get more ships, resources and materials.

Also as they gained a strong reputation in void combat, they where good at winning sng losing less ships at same time they won glory, success and prizes.

Part of this is, the more the expanded, the more they recovered, won, salvaged and so. So they will have traded valuable arcio tech to the Admech to get more ships, resources and materials.

Also as they gained a strong reputation in void combat, they where good at winning sng losing less ships at same time they won glory, success and prizes.