Excel File Size

I have an Excel 2003 workbook that has 22 worksheets. Some have smaller sets
of data and some have over 1000 rows and 80 columns. The workbook does have
pivot tables and many formulas in it such as IF, INDEX/MATCH, SUMIF, AVERAGE,
AVERAGEIF, etc. There are no VLOOKUP formulas as they take up much more disk
space than INDEX/MATCH. However, my file is still 20639KB with January's
data. I'd like to reduce this as this file is used often and will grow as
one month's business increases. Does anyone know what causes the size to
grow? Perhaps I can change out some formulas or something else that can
reduce the file size. Thanks.