Good thought. When I remove the imbedded chart each sheet gets smaller by 4K bytes. Restoring all the formating to General, no fill, no borders, automatic font color, default column widths and row heights each sheet gets smaller by 3K bytes. The saved file is still 11,978,762 bytes with one page and 12,893,184 with two sheets.
I can zip the one sheet file to 768,165 bytes (including the 116 byte zip overhead) and the two sheet file to 936,034 bytes. That leaves the 11MB I can't find at it compresses to approximately 600KB (95%) which means its not null but contains more than an empty workbook.
A completely empty, one sheet file is 13,824 bytes and compresses to 1179 bytes (92%).
I thought it might be expanding internal table information based on the number size of the matrix represented by the highest active cell but a single character in r1000c100 or in r2000c200 is the same size as an empty workbook.
Still looking for an answer. This thing raised #$%@ on the email network if its mailed without compression.