Try selecting all of the cells that are blank (through the entire worksheet - or worksheet by worksheet in a large workbook) and right-click on one of the selected cells and click "Clear Contents". More than likely if you clear the contents of your 'blank' cells, your worksheet size will decrease dramatically. Sometimes your cell formatting can get a little out of control with a wrong click or two, which causes formatting to be applied to the entire columns/rows instead of just the ones you want formatted.
