Simply because the size of an atom is determined primarily by
the number of electron shells, and electrons (which make up
electron shells) are so lightweight that they have negligible mass
(ie. approximately 1/1840 the mass of a proton/neutron).
Note :
Of course, 1 can argue that the heavier the atom, the greater
the no. of protons, hence the greater the no. of electrons and
electron shells, hence the greater size of the atom. But since
the OP/TS's teacher is obviously trying to teach his/her class a
simple point, so let's not split hairs here...