I just looked at your character in the vault with LETO, as I was concerned it was still stuck with the "mounted" appearance. It's set with the correct appearance for your character's race, which should bring with it the proper collision circle as well. If you're not seeing it improve with dismounting and/or relogging, my feeling is that the NWN engine is inconveniently keeping some of the appearance size data from the mounted appearance when dismounted.
Please keep an eye on it and let me know if it actually improves.
