  High-precision knowledge of electromagnetic form factors of nuclei is a
subject of much current experimental and theoretical activity in nuclear and
atomic physics. Such precision mandates that effects of the non-zero spatial
extent of the constituent nucleons be handled in a manner that goes beyond the
usual impulse approximation. A series of simple, Poincare-invariant,
composite-proton models that respect the Ward-Takahashi identity and in which
quarks are confined are used to study the validity of this approximation. The
result of all of the models is a general theorem showing that medium
modification of proton structure must occur. Combining this result with lattice
QCD calculations leads to a conclusion that a bound proton must be larger than
a free one.
