That is probably not easy with the online editor.

The most straightforward way would be as such: Instead of uploading each area separately, continue editing the neighbour, then another neighbour and so one.  Only upload once at the end.  This will generate one single changeset instead of a bunch of small ones.

If you select the areas wisely (i.e. all the areas of the same province), the resulting bounding box will not be too large.

Have a nice day.