Vacuum suggesting doubling of max_fsm_pages

Hi folks I'm hoping somebody can demystify something for me.

I've got a test database that is going live.

During development I have not vacuum'd much, so to get started I did
"Vacuum verbose analyze".

At the end of the process it told me that vacuum needed max_fsm_pages to
be set at about 212000 (appx) to track all free pages.

I then ran it again (w/o yet changing max_fsm_pages) and it came back
asking for a number in the range of 490000.

So I set the value to 500000, restarted the server, and ran vacuum
again. This time it suggested a value of 980000.

It appears to me the requests are doubling in size, so before increasing
it again I thought I would ask here.