Is your data inserted each time? This could cause unnecessary leftover cells with *stuff* (formatting, etc.) being saved in them. To test, open and click on A1, hit Shift-Ctrl-End. Is more than your data selected? If so, delete.

Are you building up formats in your file? Always format by grouping as many columns, rows, cells as possible, then apply the desired format. If I format 10 cells individually, Excel saves 10 formats. If I format 10 cells at once, Excel saves 1 format.

Also, copying sheets to a new workbook (instead of copy/paste cells) sometimes gets rid of potential corruption, though I consider this less likely in your case.

Just some thoughts...