Saved file size not reduced to original

I have an Excell 2003 workbook with 6 worksheets and some VBA programmed macros in it. With a small amount of data the .xls file size is 150k. Then I tested it with a much larger sample of data, which goes into one of the worksheets, resulting size 3MB. Later on I reverted to reading in the original small amount of data but the file size stubbornly sticks to 400k instead of reverting to 150k.

The old data is definitely not there anymore - I used this code to clear it:

  wsDATA.Range("A" & DATA_HEADING_ROWS + 1).CurrentRegion. _
Offset(DATA_HEADING_ROWS, 0).ClearContents

On further checking if I press ctrl + End it shows me a cell way down where the end of the old large data was. It seems I haven't cleared everything. Any suggestions as to how to clear it all?