Index bloat in 7.2

I've got a box running 7.2.1 (yes, I know :-() in which an index for a
rapidly turning over (and regularly vacuumed) table is growing steadily in
size.  The index in question is on a timestamp field that is just set to
now() on the entry of the row, to enable the query that clears out old data
to an archive to run efficiently.  Reindexing shrinks it back to a
reasonable size. Other indexes reach an equilibrium size and stay there. The
behaviour is fine on a system running 7.4.x: the index stays at a sensible
number of pages.

Is this likely to be related to a known issue with 7.2 that got fixed, or
have I got potentially more serious problems?

Thanks

Julian Scarfe