big pg_attribute_relid_attnam_index files

hi all... i was peeking in the files of postgres ( 7.2.1 ) and found a 700Mb 
file . looking in the system tables, i found that it corresponds to the index 
pg_attribute_relid_attnam_index. Also, pg_attribute_relid_attnum_index file 
is similar. I set up another db usinc rsync, dumped the db, droppped it, and 
restored. now the 2 files look normal. 
I run vacuum --analize about 3 times a day. why that files become so big? 
there is a solution, other than dump & restore ?
TIA 
Ivan
-- 
Lucy:	Dance, dance, dance.  That is all you ever do.
	Can't you be serious for once?
Snoopy: She is right!  I think I had better think
	of the more important things in life!
	(pause)
	Tomorrow!!