Or is it the same as a petty soul gem (petty)?

Size of the soul matters, not the gem.

No a mudcrab soul is just hogging a valuable soul gem and its wasted.

You would think that Bethesda would have figured out some sort of partial-fill system for the soul gems by now (since the Oblivion modders did for Oblivion, after all).

Sometimes I wonder why Bethesda keeps cutting things out of the game without putting something new in to replace it. Its not like less content is helping them focus any.

Purportedly, dropping a soul gem thats not at full capacity will clear it, haven't tried it myself.

http://www.gamesas.com/topic/1300172-rel-smart-souls/

PC only of course.