Quite a nice piece of work you've got here, although I don't know how much mobility that setup can really have. Does the pistons at the back of the knee have problems with clipping if the angle becomes very tight?
Mecha is a term used for all sorts of technical/mechanical sci-fi vehicles, its a Japanese term. However, the Term Mech originates from BattleMech aka Mech Warrior. I put these "mobile armors" into the following categories:
Ultra Light Powered Armor: stuff like nanosuits etc.
Light Powered Armor: Hardsuits etc.
Power Armor: everything human sized and powered by hydraulics.
you can then put the Power Armor into light, medium and heavy categories if you want to.
Landmates: they are power armors but the setup and sizes is different. Well known thanks to Ghost in the Shell and Appleseed.
Everything after that would be called "Walkers" or "Mechs." The definition of a Walker or Mech is, that it has cockpit for the pilot, stands on 2 legs, is between 7 and 15 meters tall and weights between 20 to 100 and more tons.
Everything beyond that would go into the Giant Robot Category with stuff like Gundams etc.
chest seems a little oversized. especially when taking the rifle into account. Unless there's a scope in the helmet, it would be very hard to take sniper kills. even as big as the Barrett M105 may be, compared to your armor, its partically a peashooter. solution: bigger and more more bad ass gun still, EPIC WORK!
I love the photorealistic render. Though it the suit seems disproportionate for a guy to fit in there, especially the helmet. I'll just wait for the diagram then. Try adding some chromatic abberation in Photoshop's Lens Correction filter for even more photorealism.