In order to design the ideal sci-fi powered armor, one must first determine the building codes of the planet being invaded.
The reason is simple - infantry need to be able to go inside buildings. War aims are frequently found indoors. Powered armor that is too big or heavy to go inside is just a very small vehicle. So when designing your futuristic sci-fi toys, you need to take into account how much weight the average stairs can hold and how wide and tall the doorframes are. The cramped hallways of a space ship during a boarding action will be even more limiting.
This frequently stretches my suspension of disbelief in 40k stories. Perhaps the Imperium's architecture takes into account that an 8-foot tall primaris marine bedecked in Gravis or Terminator armor might pass through, but there is no reason why alien civilizations would make their structures navigable to such giants. If they are aware of Space Marines, they might intentionally construct cities and fortresses to frustrate oversized invaders. Marines would be forced to come in through the loading dock.
Speaking of loading docks, armored vehicles are likewise limited by the roads and bridges of the cities they will be operating in. People and things tend to be in cities, so war aims are almost always inside cities, if not the cities themselves.
It therefore follows that before you can start designing, testing, and manufacturing the equipment you will use to conquer a planet, you need to know where you will be fighting.
This isn't a problem in Dropfleet (where the main invasion targets are conquered human colonies) or Battletech (where architecture is reasonably standardized throughout the Sphere), but again stretches credulity in the diversity of 40k.
One possible way around this would be to have various sizes and weights of vehicle and only bringing those supported by local infrastructure. Battletech has explicitly done this in some video game missions as a rationalization for tonnage limits.
There was one 40k story I remember where Space Marine in terminator armor broke the floor of the building he was in and had to wait prolonged period of time for heavy equipment to get him out of the cellar. I think this was the only time I remember when it was actually addressed by Black Library. I also wish it would come up more often.
