Im not sure EXACTLY how this plays out but keep in mind the differences in sheer mass when looking at crash test results.
I.e., im pretty sure a 2500 lb car rated 'good' and a 4500 lb car rated 'good' are not equally good. The difference between the smaller minivans and the larger minivans is close to 1000lb and the bigger ones usually give you several more inches of crumpling metal between you and outside world in every direction.