Personally I associate PM Optimus as only being the super mode, more than I do Ultra Magnus. The individual Optimus toy was bad, even back then, and he never used it in fiction. You can *almost* transform him directly without taking the cab off the trailer.
UM though was almost exclusively a toy, since once the cartoon was off air, that's all there was for him over here. It also made him extremely unique. Supermodes and armore upgrades were unheard of back then. UM was the first. Optimus was the second. And that was pretty much it. So for me, they sacrificed one of THE most important parts of UM to go straight into super mode. I can deal with the same thing from Optimus far more easily, since because of fiction, the supermode is what made PM Optimus special.
That said, a bit heavier retooling COULD include a deluxe sized G1 optimus. It honestly wouldn't be that much more difficult. Though by then we're approaching the point where it's better to just design one from the ground up. Which is also why I was upset over the UM they gave us being IDW Minimus Albus based.
Yeah, I'd agree - although the smaller robot mode for Powermaster Prime was indeed one of the selling points (and also something I remember liking at the time, since he was more cartoon accurate - which is to say, accurate to a badly-drawn cartoon), aside from an appearance in The Cosmic Carnival, Prime's smaller robot didn't really show up at all. So I wouldn't be unhappy.
Except from the point of view that power-up suits for smaller robots are always cooler than just having a large robot. ;-)