Does anyone have any suggestions?

I get a table A1:Z4000, when I select this range and replace $L with $U, the
file size increases unreasonable large, but when I only select the first row
and replace $L with $U and copy it down into row 4000, the file size only
increase a little bit.
Does anyone have any suggestions on what cause the file size increase by
replacing $L with $U for the whole table?
Thanks in advance for any suggesitons
Eric