size of excel file saved in 2003 format

Hi -

I am using Excel 2007.

I have a workbook with several sheets. There's not much data, but there are a few small macros and formulas in one sheet which refer to another sheet. (and when I say not much data, I mean like 10 rows of 15 columns on each sheet).

When I save it in native 2007 format (xlsm), the file takes up maybe 500KB.

When I save it in a version that office 2003 can read it, it takes up 27MB!

Is there a reason for this? Is there a way to change that?

We feel like we need to offer 2003 format as we are sending this spreadsheet to many schools and we don't know if they've all upgraded to 2007 (or 2010) yet.

Thanks!