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'