Fake it 'til you make it!

The square-cube law is about how increasing the size of an object increases its volume much more rapidly. So if you make an ant, say, twice as large, it ends up 4 times as heavy (don't take these numbers as anything but an example, I'm pretty sure there's formulae and shit). For that reason, massive vehicles, like mechs, are impractical - something twice as large as a tank is gonna end up much more than twice-as-heavy.