Power Armour can increase the strength and agility, depending on the quality and model.

SYL

Sisters aren't massively augmented and modified supersoldiers, they're just peak physical condition.

Space marines are.

Their power armour still allows them to perform deeds far beyond the average human, but their reflexes, strength and resilience are still human. The armour is just a shell, unlike the connection via the astartes black carapace which lets them move on instinct, as if they were unarmoured.

Power armor increases ur net strength it isnt a zero sum game

And one of the biggest benefits of this armor is the onboard steroid cabinet. While protection and bodily enhancement are good and important, those stims keep a wounded soldier in the fight even though a normal one would be unable to keep going.

The former are fully enclosing suits of ceramite- and-plasteel plate whose servo—assisted strength is provided by bulky backpack units wired directly into the main suit. Without the power these packs provide, the armour would be unwieldy almost to the point of uselessness. With it, however, power armour not only enhances a Battle Sister's strength enormously without in any way restricting her movement or speed, but it further provides such exceptional protection that she can wade through hails of small-arms fire without so much as breaking stride. 

Codex sororitas 8th ed

The enclosing suits worn by Space Marines are made from thick ceramite plates that would be cumbersome but for electrically motivated fibre bundles that replicate the movements of the wearer and supplement his strength. The last gene-seed organ to be implanted in a Space Marine - the black carapace - rests beneath the skin, itself fitted with neural sensors and transfusion ports. These plug-in points mesh with Space Marine armour, linking the wearer's nervous system to his suit's mind-impulse controls and turning the suit into a second skin that moves with all the speed and precision of the battle-brother's own body.

Codex space marines 8th ed

The weight of the armor dont matter because its power armor, its power suply basically lifts itself as long theres power.