  Accreting main-sequence stars expand significantly when the mass accretion
timescale is much shorter than their thermal timescales. This occurs during
mass transfer from an evolved giant star onto a main-sequence companion in a
binary system, and is an important phase in the formation of compact binaries
including X-ray binaries, cataclysmic variables, and gravitational-wave
sources. In this study, we compute 1D stellar models of main-sequence accretors
with different initial masses and accretion rates. The calculations are used to
derive semi-analytical approximations to the maximum expansion radius. We
assume that mass transfer remains fully conservative as long as the inflated
accretor fits within its Roche lobe, leading stars to behave like hamsters,
stuffing excess material behind their expanding cheeks. We suggest a physically
motivated prescription for the mass growth of such "hamstars", which can be
used to determine mass-transfer efficiency in rapid binary population synthesis
models. With this prescription, we estimate that progenitors of high-mass X-ray
binaries and gravitational-wave sources may have experienced highly
non-conservative mass transfer. In contrast, for low-mass accretors, the
accretion timescale can exceed the thermal timescale by a larger factor without
causing significant radial expansion.
