"small" doesn't tell you much, a 32A is much much smaller than a 38A.
It also depends on skin elasticity, how stable is the person's weight,
the resiliency of the cooper's ligament (which is hereditary) and
the resiliency of the breasts connective tissue, also hereditary.
Also depends on activity.
Someone who's a 38A, takes a lot of sun and has bad support
from the internal breast structures due to heredity, who
plays basketball and has a yoyo weight has a near 100%
chance of sagging early.
A 32A who doesn't wear a bra and has none of the
risk factors mentioned stands a good chance of
being very perky for a very long time.