They have relatively small undergraduate classes. I bet if you took the top 1/5 of a typical BYU class it would compare well with a typical Ivy institution in both size and student quality.
If BYU capped its undergrad enrollment at 1000 per class its stats would be crazy high, too. But BYU has a different goal than Ivy schools. Quantity is important to BYU.
FQIW, My wife transferred from BYU to an Ivy school and she didn't think the Ivy students were much different., except for amount of alcohol consumed and political views.