Hi Brian,

Have you checked for extra columns / rows that don't need to be there?
Check if your scroll bars are particularly small as this is a sure giveaway that you do have these extras.

If you accidentally move all the way down to the last row (1,000,000+) and put a space in one of the cells down there, the spreadsheet will assume you're using all the space down to that row and this will drastically increase the file size. Same could happen with columns.

Try scrolling to the last row you're using and highlighting and deleting all the rows below it. Then select a cell near the top and save the file.

If it's not that, then the only other things I can think of would be excessive conditional formatting, change history or data ranges in other workbooks.
These I find are the usual suspects for increased file size.

Hope that helps...

S. :)