I use a lot old (or not that old) artwork into some new client jobs (eg: a logo or a background for a new project for consistency purposes) and sometimes embedding prior illustrator files in illustrator creates bigger artwork than original. (when I mean bigger is size i'm talking about. eg.: a square in original file was 10x10mm and when embedded gets 13x13mm)

It happens only when embedding and the only workaround is using flatten transparency.

I still don't know what exactly triggers this issue as it happens with seemingly random files.