Independent secondary schools range in size from as little as 150 pupils to over 2000. Just as some people are happier in the bustle of large cities than they are in small towns, some children will do better in a school of a certain size. Consider the facilities and subjects offered; there may be more going on at a large school and possibly newer, more extensive facilities. But smaller schools may be better at encouraging pupils to be involved in what is available.
Large schools can often give pupils some of the advantages of small schools by breaking down the whole school into houses, with a community life of their own. It does not necessarily follow that because a school is small it has a more caring environment than a larger one.