  We compare form factors in sine-Gordon theory, obtained via the bootstrap, to
finite volume matrix elements computed using the truncated conformal space
approach. For breather form factors, this is essentially a straightforward
application of a previously developed formalism that describes the volume
dependence of operator matrix elements up to corrections exponentially decaying
with the volume. In the case of solitons, it is necessary to generalize the
formalism to include effects of non-diagonal scattering. In some cases it is
also necessary to take into account some of the exponential corrections
(so-called mu-terms) to get agreement with the numerical data. For almost all
matrix elements the comparison is a success, with the puzzling exception of
some breather matrix elements that contain disconnected pieces. We also give a
short discussion of the implications of the observed behaviour of mu-terms on
the determination of operator matrix elements from finite volume data, as
occurs e.g. in the context of lattice field theory.
