lol man, if I had a dollar for every time someone tried to use physics to reason for the impossibility of my fictional machines...I could quit my job.
I'm well-versed in physics, but that's where fiction comes to my aid!
Seriously though, I know they're overly huge, but there are various reasons for it, one of them being due to the need for larger payloads (the original mecha in the story -from about 40 years ago- were about 3 meters high, more along the lines of a powered suit of some kind), and in my story they are supported by various systems and hugely different metals than on Earth. Definitely not for terrorizing cities all godzilla-like. There was only one major incident where they were used in a metropolitan area, but it was just a desperate attempt to cover something up, not part of any intended assault.
...They do not have lambda drivers, but unlike the Behemoth from FMP they will not collapse under their own weight (if that was perhaps a reference you were making?).