While stripping down an old Diamondback MTB to repaint it, I pulled the seatpost out and noticed it hadn't been very deep in the frame. Being slightly paranoid that the frame might have been damaged by the marginal seatpost insertion and me not being the lightest young lady around, I felt around the inside of the seat tube with my finger to see if I could feel any cracks. A few centimetres in, the tube diameter seems to increase by a few millimetres - I can feel a step inside the tube. I did some digging online, and apparently this could be a sleeve inside the seat tube, which was something Peugeot used on some of their old frames. Those frames, like this one, take a 24mm diameter seatpost, which lends weight to the idea.

My question is this: As the seat tube diameter increases after about 30mm of depth, is a longer seatpost going to be any use, given that it won't actually be in contact with any part of the inside of the tube other than the sleeve?