Answer
The forces are the same, but the compact car experiences the larger acceleration.
Work Step by Step
The two forces mentioned in the question, $\vec{F}_{big\;on\;small}$ and $\vec{F}_{small\;on\;big}$, are always equal in magnitude and opposite in direction. This is a statement of Newton’s third law.
The forces are the same, but the compact car experiences the larger acceleration. Because the 2500-kg car has 2.5 times the inertia of the 1000-kg car, and net force = ma, the smaller mass experiences an acceleration 2.5 times larger, because a = F/m.
The passengers in the smaller vehicle are more likely to be injured because they are brought to rest in a smaller amount of time (due to their larger acceleration).
