My guess: few(er) renters choose a large SUV (e.g. Truck-based Tahoes or Yukons) when gas prices are high, so the fleet is adjusted accordingly. There are also large (7-pass) cross-over substitutes (e.g. Enclave, new Explorer) that are also filling that segment.

In other threads, it sounds like they can't even give Tahoes away (sometimes) on ES or EA because some folks don't want to drive such a large vehicle or pay at the pump for one, even if it would be a mega-upgrade.