  We compare the properties of local spiral galaxies with the predictions of
the Cole et al. semi-analytic model of hierarchical galaxy formation, in order
to gain insight into the baryonic processes that were responsible for shaping
these galaxies. On the whole, the models reproduce the properties of
present-day spirals (such as SFRs, SFHs, scale sizes, metallicities, and gas
fractions) rather well. In particular, we find that once the effects of dust
and variations in stellar populations have been taken into account, published
spiral galaxy scale-size distributions derived from optical data (with
logarithmic widths ~0.3) can be reconciled with the width of the stellar mass
scale-size distribution predicted by the semi-analytic model (~0.5). There are
some illuminating discrepancies between the observations and the model
predictions. The model colours of luminous spiral galaxies are somewhat too
blue and those of faint galaxies somewhat too red, most likely indicating
shortcomings in the way that gas is accreted by spiral galaxies. Furthermore,
the model produces too few luminous spiral galaxies. These difficulties could
be resolved by altering the way in which gas cooling is treated or, perhaps, by
adopting a higher baryon fraction and invoking galactic `superwinds.' Secondly,
stellar M/Ls are found to be as high as observations permit. Yet, typically 60%
of the mass in the inner half-light radius of the model galaxies is dark. This
results in an offset between the model and observed spiral galaxy
luminosity-linewidth relation. This could be resolved by substantially reducing
the mass of baryons which make it into a galaxy disc (with an attendant
decrease in stellar M/L), or by modifying the assumed dark matter profile to
include less dark matter in the inner parts. [Abridged]
