Converted sheet grew in size--same data
 

I have an Excel 2007 sheet with 257k rows. It is 26.1 MB.
I "down-converted" that sheet to Excel 2003 by placing 65500 rows on
each tab. The 2003 sheet contains the same amount of data and it is
56.6 MB in size.

Why did the 2003 file double in size from its 2007 counterpart while
it contains the same amount of data?
Is there anything I can do in this process to get the 2003 file down
closer to the 2007 filesize?

Thanks,
RLN