@Stephen Sutton and @Charles Tottington - I think the anatomy involved in this sort of thing is maybe something best just handwaved: I've spent too much time trying to reason through how your legs and hips are supposed to fit in this sort of armor while still being something you can walk in, or what happens when you end up with an itch that needs to be scratched or sweat in your eyes, and realize it's best not thought about, better just accepted in broad strokes.
The larger mecha side of this spectrum - vehicles with cramped aircraft-style cockpits for people to sit in - probably make more sense in terms of practical real-life human(oid) ergonomics (see Japanese-style mecha, Battletech/Mechwarrior, the Imperial walkers from Star Wars, and the Martian war machines from War of the Worlds), as do remotely-controlled drones, artificially intelligent androids or robots, or, in the more dystopian sci-fi angle, installation of a surgically or genetically simplified human body - such as just the brain - into an android, biomechanical, or mechanized body (such as Robocop, Doctor Who's Daleks or Cybermen, or those killer silver spheres from the Phantasm movies!)
Alterntaively, I take it that the basic power armor idea works in the Warhammer 40K universe in spite of the grotesque proportions because the trans-human inside the armor is grotesquely proportioned: genetic engineering and so on result in freakishly wide shoulders and hips, big arms, and so on - space marines can wear that stuff comfortably because they aren't shaped like human beings anymore, they're freakishly-proportioned, genetically-tampered, biologically-engineered, super-soldier mutants!