The size of the saved workbook is entirely dependent on what it contains, not merely the area of the used ranges on each worksheet. Complex formulae, graphics, macros & userforms, hidden worksheets, tracked changes, etc, can all lead to a workbook being much larger than the visible data might suggest. I've used workbooks of 40Mb that had no data as such - just formulae for processing data that could be pasted onto a data sheet. As for:
Excel doesn't save files by 'tab'. If what you mean is that you copied what you considered to be the used range on each worksheet to a new workbook and saved that, the process you went through may have omitted content that Excel was holding in the source file and/or lead to Excel only saving the data or a truncated range. For example, pressing Ctrl-End on a worksheet might take you to a cell that's well outside what you consider the used range.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]