Found the solution:
http://support.microsoft.com/?id=244435


"ka" wrote:

Hi
I created an excel sheet and copied&pasted a few images from a pdf file. I
later noticed that the file size got quite big (35MB). So I deleted all the
images, saved the file, and...the file size is still the same! Without the
pictures, the excel sheet used to be 1MB. So, how do I bring the file size
down again?

Any help would be appreciated.

Best regards