The original question brought up by the OP doesn't need asking. The first question that should have been asked, if you really want to bring the mythos and compare it to modern science is: why are we building giant robots with legs, when tanks on wheels or with hover jets are much more useful?
We probably could make a small, hydraulic mech in the current day. It's just impractical. The center of gravity for one of these mechs in comparison to a tank on land is so much higher off the ground that any kind of concussive projectile would send your mech straight on its butt, effectively destroying its combat capability.
I'm an engineering student, and I agree that 100 tons for a mech of this size and loadout is questionable, but the idea that a military would consider a mech a more effective war machine in the first place is ludicrous. We play mech games because it is awesome to see giant robots fight each other. But realistically, they're impractical, slow (for the amount of power it would take to move their legs), and ineffective against tanks with lower centers of gravity that take a heck of a lot more force to flip them on their back sides.
EDIT: changed for clarification and reference to original topic question.
Edited by daricsoldar, 27 June 2012 - 06:00 AM.