Just to clear up your terminology: The Seat Tube is the tube of your frame running from the bottom bracket up through the seatlug. What you are calling your seat tube is referred to as the seat post, or pillar.

By change shape - is there a ridge? or are you just saying it necks down? Is the tubing out of round, say, from the improper tightening of a workstand clamp? You might need to check roundness with calipers.
A ridge actually be remnants of a former seat post - or possibly an indication of some kind of repair. Some manufacturers inserted shims for use of smaller seatposts. I have no idea if Rauler did that, and in any case, a shim should come to the top of the seattube.