Mozart's Orchestra
Mozart's orchestras were quite small by today's standards. The scores for his orchestral works often include no percussion at all, and when they do usually just the tympani. We can list several reasons for his small orchestras:

Musical Instruments in Mozart's Time
In the 18th century many current musical instrments had not yet been invented. These include Many of the instruments that Mozart did have were significantly different, and have been much improved today.