In my research field (CS), large universities have a big advantage. They have more resources (valuable for systems fields) and more students. Even if Harvard ma

In my research field (CS), large universities have a big advantage. They have more resources (valuable for systems fields) and more students. Even if Harvard may have stronger undergrad students overall than random-U, their candidate pool of undergrads->potential PhD students tends to be much smaller. This isn't absolutely a rule (some smaller schools do very well in specific areas) but it's definitely a force I've noticed.