Excel file size increases when saving - 80KB file becomes 26MB, help!

When I save an excel file of normal size received from another user (via email or off my company's share drive), modify it using excel and resave it, the file size increases significantly. This is quite a hassel since I can then no longer send it back via email with ease.

For example: a 80KB file sent to me via email turns into a 26MB file when I modify it and save it to my desktop

This does not happen when I just save the sent file to my desktop. This only happens after I make a change, no matter how insignificant the change may be. This happens with ALL documents, not on occassion but ALWAYS.

Please help!

I use Excel 2010.