Export video has different resolution Adobe Premier Pro
Exports will be set to the resolution of the sequence you are exporting.  You need to adjust your sequence settings to match your file input and then export.  You can do this from the sequence settings.  It should have also given you an option to update the sequence when you initially added the 1080p clip to the sequence, but you must have chosen not to update your sequence at that time.

The other, more rare, possibility is that you are using a demo codec that doesn't support 1080p for export and it is trying to export at the maximum supported resolution.  If this is the case, choosing a different codec for export should allow you to resolve the problem.