Why are the bottom teeth smaller than the top teeth?

Answer 1

Due to small arch (lower arch in ur mouth is smaller than the upper )

and to capable the teeth upper and lower to rest on 2 teeth not one ( i mean like upper central incisor will rest on lower central and lower lateral incisor and so on).

Formally answer :

Due to convergence of lower arch (as lower arch is more convergence than upper arch)

Answer 2

That is not the case in everyone's mouth. Some people have more out of place upper teeth.