When you squeezed in on the bottom. the bottom where you squeezed got skinner than the top, since you did not remove any metal this caused the area that did not get the pressure from the vice to get wider than the top.
the idea is the top did not change shape and is still round, so by pushing the way I have it, you are using the top as a guide to push the lower part back even with it.
By pressing that wider area back even with the top the skinnier area will push out slightly. I'm guessing since this is brass the metal wont have a lot of spring to it. If this was SS you probably have to push the wider a bit further than even wit the top.