formula/function usage creating huge file size?

I have an Excel 2000 spreadsheet that uses VLOOKUP to
equate text values (a drop down validation list) with
numeric values (to calculate a score). The workbook has 5
tabs but none of the spreadsheets are especially large.
Each row has 3 VLOOKUP instances. But the file size is
13Mb. My PC can barely process the file it's getting so
big.

I am not using any VBA functions, and I ensured that no
Macros exist. All I'm using are data validations, VLOOKUP
and some simple calculations based on the results of the
lookup.

Any hints on what is causing the huge file size and what I
might do to minimize it?
Thanks, Walt