I'd agree, by the time the KOTOR games are set in Ossus is pretty much a graveyard after it was pummeled by Exar Kun. But there would be the ruins of the old academy and ancient jedi library there. It is basically like the Lightsided Korriban. I think it would be a good planet to visit.
As for the concept of bigger planets, I personally get incredibly sick of a planet after too much time on it. I personally feel that Korriban was a suitable length for the context of the planet, it served its purpose and if it was any longer I feel it would have been pushing it. Storyline wise told us what