What is your understanding of why he chose to do this?
The Pieta shows us a Mary who is considerably larger than Jesus. As far as I know, Michelangelo didn't give us his reason for this. I will only guess that Michelangelo didn't make a mistake but rather chose to portray a small, dead and limp Jesus laying across His large mother -- to contrast Jesus' weak humanity and the payment he made for sin against a mother displaying her strength and love in the face of death.
My art history teacher said that Michelangelo made Mary look larger, with massive drapery around her head and shoulders, because he knew the statue would be placed up high on a pedestal, but viewed by visitors from the floor below.
With the centuries-later advent of photography, we see pictures of the work straight on. Michelangelo’s masterful manipulation of space does not work at the full frontal angle.