If I set up a layout with a detail, scale that detail to a specific scale (this will be most noticeable if you are zoomed way in or out, so that you can see quite a bit beyond the extents of the model space objects in the detail). Then with the detail active and unlocked, change the size of the layout sheet. Over here, the detail zooms to fit. It doesn’t seem that details should zoom under any circumstance unless explicitly done by the user.
Thanks,
Sam