It would seem the most likely cause is the addition of cells either by formatting or bad data input. An easy way to see this is by looking at the scroll bars. If the drag bar is extremely skinnny and you are only using 100 rows (or A-G columns) then you have a lot of extra rows/columns being remembered. Copy/pasting the required data into a new workbook will fix this but may cause problems depending on the type of data involved. A way to remove the hose unnecessary cells is by dragging the scroll bar to the end (don't use the scroll wheel) and then selecting and deleting the "extra" rows or columns.
Check the reviewing options as well. If the document has been passed around a few times and markups have been applied with out a full accept/deny of those changes, the document can get very large. This is more common with PowerPoint and Word but can affect Excel.