Prophecy2k
Veteran
Armor for Space Marines was closer to medieval plate mail in that it basically just protects the user, granting no superhuman abilities above and beyond those already possessed. Thus Platemail would be a more apt description of the origin of Space Marine armor. Now Terminator armor on the other hand. That could potentially trace it's roots back to Starship Troopers armor. But again only superficially, as it still mostly was about even more protection for the wearer, but in so doing became so heavy that even genetically altered Space Marines required some help in making it move. Being so heavy however, also meant they were slow and cumbersome (compared to standard armor), and if it's going to be slow and cumbersome why not mount extra heavy weapons?
I believe that the bolded are not entirely accurate to a degree. Mk II (i.e. armour worn during the Horus Heresy) and upwards Space Marine Power Armour does indeed augment the Adeptus Astartes already super-human abilities to even more superhuman levels, not by much but they definitely do. It's the very reason that despite the bulk and weight of the Power Armour itself, an Astartes can move and react as quickly, if not quicker, than if he is completely without his armour. The Power Armour isn't just plate armour but is an effective mechanised battlesuit, similar in kind to the mechanised exsoskeletons being made today, only full body with vox commuicator, air filtration and re-circulation systems, temperature regulators and battle/field drug administration systems integrated.
Also, Terminator Armour also isn't anything remotley close to being "slow and cumbersome". An Adeptus Astartes could outrun a car at 40mph wearing terminator armour. The suits augment an astartes abilities even further. They are "slower" than standard power armour, but they certainly aren't "slow" in absolute terms... if they were then a Terminator Assault squad would be useless... without or without their personal teleportation devices ;-)