Problem with large tuples.

I've noticed that some of my tables with large text tuples have
problem when these exceed some certain size. I know about the
8k tuplesize limit, but it seems the problem appears earlier
than that. This is what I've been able to recreate:

CREATE TABLE sizetest (id int, txt text);
INSERT INTO sizetest (id, txt) VALUES (1, '...'); // ... = string of
4000 chars
vacuum analyze; // <-- works nicely
INSERT INTO sizetest (id, txt) VALUES (2, '...'); // ... = string of
4100 chars
vacuum analyze;
ERROR: Tuple is too big: size 8152, max size 8140

How come the insert works while the vacuum fails?

Setup: Postgres 6.5.2, FreeBSD 3.3

/Patrik Kudo