They could easily justify the BW characters not needing to scale to the original animal size though. There was no reason for them to hide their forms as at the time the planet was only inhabited by other animals and cavemen, which we only saw briefly. It's quite obvious that scale wasn't an issue in the original show, unless Rattrap scanned a R.O.U.S.
Long story short, if there's nothing to hide from, no need for realistic sized animal modes.
I also find it odd that the Japanese didn't like Transmetals. Personally I find organic beast modes extremely boring, but when I suddenly had myself a purple mech gorilla? - Hells yeah! Maybe people in Japan are just too spoiled for their robot action.