  Estimates of the linear extent of heavy-element absorption systems along the
line-of-sight to a QSO often assume that the cloud is photoionized and that the
temperature takes the equilibrium value where photo-heating balances line
cooling. We show that rather small deviations from this photoionization
equilibrium temperature caused by additional heating processes will lead to an
overestimate of the neutral hydrogen fraction and thus to an underestimate of
the thickness of the absorber by about two orders of magnitude. Such
temperature deviations are indicated both by observations and numerical
simulations. This interpretation reconciles the discrepancy between the rather
small extent of heavy-element-absorption systems parallel to the line-of-sight
obtained from a standard photoionization analysis and the much larger
transverse sizes estimates inferred from the observation of common absorption
in the spectra of close quasar pairs.
