Ugh, no wonder the Shadow Cat is such a pain in the ass to kill; it's the smallest mech in the game, yet has 1k more armor points than the larger light mechs.
It's not the fact that the Shadow Cat is too small - it's that the lights are too big. Compare the Shadow Cat to the Bushwacker and you see what I mean. The torsos of both are actually kind of the same size. I'd say the Shadow Cat is spot-on for its weight.
The Osiris, Kit Fox, Owens, and Raven should be shrunk to be far closer to the Adder in height, I don't think anyone will disagree here. I remember the first mech you come across in MW3 was an Owens, and the thing was quite small and spindly. You start the game in a Bushwacker, I do recall it being considerably smaller than myself.
Actually looking at the picture again, I would even say the Adder could lose a couple of feet. It's supposed to pretty much be the lowest mech in the game, it's actually a tiny bit taller than the Cougar.
THIS should be what the first part looks like. First four mechs shrunk, Osiris and Owens shrunk the most. Adder shrunk a bit to be 'lower'. Hollander shrunk a bit to not be so damn tall. Shadowcat grown a bit. Uziel shrunk significantly. Bushwacker and Cougar unchanged.

By the way I threw up a better Shadowcat to Kit Fox comparison, with the Scat merged inside the other so you can see exactly how much bigger it is.