Space marines, by their very name, seem like they would be made to board starships and that is a big part of the fluff.
So why would you make them big, it would seem like economizing on size would be pretty key in building a space ship. After all, even if the materials are free, it is that much more air you have to purify, keep warm etc.
And even if human spaceships have been designed to be these massive space born leviathans, other races wouldn't necessarily have the same philosophy.
And the Space Marines were not intended to board ships of the nascent Imperium after all. It just so happened that the story line got really interesting when they did start doing that.
Making them stronger, putting them in tough armour, giving them extra organs and enhances senses all is logical but bigger . . . not so much. At least for boarding actions, planetside it does make more sense.
