The outer electrons in a large atom don't "see" the entire nucleus because it is screened by the inner electrons, making the net nuclear charge rather small. This is known as the "shielding effect". In other words, the attractive force from the nucleus is somewhat balanced by the repulsive force from the inner electrons. It is relatively easy to remove outer electrons from a heavy atom such as uranium.
Work Step by Step
However, it is very difficult to remove the inner electrons. They feel the full attractive force of the nucleus, and one must supply a lot of energy to strip them from the heavy atom.