However, 3TB does sound excessive for metadata. Your DF output will however only show online volumes. While it sounds like this is a new aggregate, do you have any other volumes not online that could account for the difference?
Yep, some SnapMirror destination volumes are deliberatly created large to avoid the issue when the source volume is autosized. Unless the SnapMirror is broken and the fs_size_fixed option is turned off it will not actually consume this space.