Shapeshifter, I've ridden ATC 70s, 90s, 110s and a 125M once. It's been awhile, but I believe the external size/architecture on the 90/110/125M motors is pretty much the same...big and heavy like what would have been in a OHC C90, CT90, S90, CL90, etc.
In overall vehicle size, the ATC70 is a LOT smaller than the 90/110/125M.
There were a bunch of 90s and 110s amongst my riding buddies when I was a kid, leading me to think it would be a good idea to buy a used KLT110. As it turned out, I was wrong about that, but the 90 and 110 Hondas were a good all-around size, and had a lot better transmission longevity than my KLT did.
But you could probably get away with being more stupid and less squashed on a 70.
The 125M seemed heavier, clunkier and piggier than the 90s and 110s I'd ridden. Maybe more appropriate for farm/utility use than stupid ATC tricks.