Newton's 2nd Law.
This is because Newton's second law sates F = ma or Force = mass(acceleration). Therefore, if you were to throw a baseball and a softball with a force of 20, bu the baseball weighs 10 and the softball weighs 20, the balls acceleration would be greater:
Baseball:
F = ma
20 = 10a
a = 10
Softball:
F = ma
20 = 20a
a = 1
10>1, therefore the baseball's accel is greater than that of the softball
I hope this helps!