excel 97 file grows for no reason

Well ...... I should say for almost no reason;

We have a few excel files shared amongst @12 users. Seems
as if the file size grows for no reason.
I say this cuz when the worksheets are coppied into a new
workbook, file size shrinks by @ 90%.
I find this out when some users (but not others) are not
able to open the workbook (it appears to open, but flashes
out immediately - only see it for a half second before
it's gone).
Any help appreciated!!!!!!!!!!!!!
Thanx!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!