The largest RSS (according to top) for a pg process is 298M, most of
that (295M) from the SM segment. However, top (and proc/meminfo)
claim that 360M of memory is used as fs "cache." That makes 658M from
these two sources alone on my 512MB machine. (80M of swap is used,
but neither RSS nor cache should be counted as swap, and in any case
80M isn't enough to explain the 146M difference.)
I've heard top isn't very accurate, but I don't know where else to
find RSS (or SM size, for that matter)...
