Why is it so much easier to remove an electron from an atom of a large atomic mass than it is to remove a proton?
Electrons in higher orbitals are easier to remove than lower orbitals. Large atoms have more electrons in higher orbitals.
The Bohr model of the atom has a central nucleus of protons/neutrons and an outer cloud of electrons swirling around the nucleus. In the atom's natural state, the number of electrons exactly matches the number of protons in the nucleus.
These electrons swirl around in discrete orbitals of increasing distance away from the nucleus. We denote these orbitals as s, p, d and f with s being closest to the nucleus and f being farther away.
Each orbital can only contain a limited number of electrons, so for atoms with a large number of protons, the electrons must occupy the orbitals further away from the nucleus. The farther away an electron is from the nucleus, generally then, it will be easier to remove from the atom.