Downsizing operas was customary during Mozart’s time. The so called “Harmoniemusik” (an arrangement of operas or famous arias for a smaller group of musicians), allowed the aristocratic households to entertain their guests at banquets, hunting parties and other court activities with the latest operas and arias.
For the most part, the composers themselves did not make the arrangements, that was a task reserved for other contemporaries, who were specialized in “Harmoniemusik”.