[Radek]

Well, usually the tiny one [img]smile.gif[/img] I don't think the problem is caused by the size of the orifice. The problem seems to be caused by some "sharp edge" in the orifice geometry.

Similar sharp edges can be seen, for example, in the Toad Village. There are places there where you usually need to jump if you want to pass. For example, the tunnel leading to the ladder, which ends in the Sacred Toadem area. You need to jump if you want to leave the tunnel and reach the ladder.
Now, you cannot jump underwater. You can only push yourself forward looking up or down, pushing a sleeping Unholy Filth in front of you, trying to pass by pushing yourself backward, and so on.
A small character can help if it is really small, for example, a pixie. Such character can try to swim near the ceiling of the tunnel and pass the edge making use of its small stature but most of classes aren't enough small for it.