Rukia had basically no power when Renji came after her in the first one, and she wouldn't have fought him anyways.

Rukia is really strong, as is evidenced by her easy defeat of the Arrankar, but I don't think she has a Bankai. Yet.

Also, it's possible that Zaraki's zanpakuto is still in its first form and has that appearance. Ichigo's normal zanpakuto was a large blade, Kyouraku's is two swords, Soi Fong's is a shorter blade, Gin's is a wakizashi, Hitsugaya's is a nodachi, etc. etc..

There are zanpakuto in normal forms that look different. Hell it's even speculated that Nanao's zanpakuto is her book.

If Rukia were able to use Bankai, I think they would have referenced that she was at a captain's level. If that were true, she would have done a lot better against the Hollow she fought when she first met Ichigo, it wouldn't have even been able to harm her (remember, if their spiritual energy is high enough, they wouldn't even be touchable by the attack of a weak hollow like that). I'd say she's pretty strong, but not at the Bankai level. Though that doesn't necessarily mean she won't use it.

And on Ichigo, Hollow Ichigo can use a lot more attacks with both the Shikai and the Bankai forms of Zangetsu, so that implies that Ichigo just doesn't have full mastery of it. Look what Byakuya was able to do with HIS Bankai after all. There's probably a lot more he doesn't know with regards to Zangetsu.