I started to make a T.A.R.D.I.S. interior from Doctor Who in SketchUp Make (15.1.104, computer: MacBook Air Version 10.8.2) and I’m trying to make the dome wall by making an arc on the floor and Push/Pull-ing it (so you can still see the interior from the outside) and change the radius of the top base, yet every time I do, the circumference does increase, but the position of the circle changes completely.
I suggest that you push-pull a complete circle into a cylinder and then change the top radius (or scale the top surface uniformly by its center, and finally erase the parts you don’t need. This keeps the centerpoints aligned. If you change the radius of an arc that has its endpoits attached, the centerpoint will move.