I believe smaller schools do better in tests because they have more individual time with their teachers. I think individual time is important to students because the teacher has more time to help their students and see what they are having problems with. The teachers also have a better understanding of where their students are developmentally.
For standardized testing, the size of a school doesn't necessarily impact its performance in relationship to other schools. A smaller school won't get better scores because of its size in most cases. The richness of a curriculum dictates how well a school will perform on these tests in relation to nearby schools no matter the size.