Since no-one here has seen the file or possesses full details of what edits were made, we can't really be expected to tell you what the problem is. If the original and edited files were the same docx file, compressing it on a Mac would have little to no effect. The mere fact that changing the file format from docx to doc reduced the file size tells us there was a significant loss of content not supported in the doc format. That's a pretty clear indication we haven't been given the full story.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]