Ubuntu's install is comparable to the size of a Windows XP install. Except it comes with a whole office suite, and a load of other apps.
As for the driver situation under RISC OS; yes. It's a complete mess.
For rewriting, loads of Arthur stuff needed a hell of a lot of reworking in their GUI code to be made RISC OS 2 apps. I'm just saying that a similar amount, if not more, would be needed again to move them onto a PMT system.