There's nothing to say that Convobat wasn't already a non-beast moded Maximal before becoming a bat. He could have still transformed into a Micromaster sized truck (like say Powertrain) back on Cybertron before adopting his batty beast mode on prehistoric Earth.
It's possible that in toy continuity Convobat remained on Cybertron and never acquired a beast mode whereas in toy continuity he did. It wouldn't be the first time it's happened... look at Cicadacon, Ram Horn and Sea Clamp (Tripredacus Council) -- their toys all have beast modes yet in the cartoon they never went to Earth or acquired beast modes. I think it's accepted that Tripredacus never had beast modes in the cartoon, but did in the toy continuity (and possible other parallel universes) -- the same could easily apply for Convobat.