Probably the biggest weakness of 5E is that it can't decide whether it is a Dungeon Fantasy game or not. Half od the design choices point toward dungeons, but the other half don't -- and the "culture" presented in the official modules certainly doesn't. You are absolutely right in your comment "needed a bigger dungeon" because the attrition of hit points and spells that "balances" fighters versus casters is built around the dungeon. The PCs should be pulling out when they are spent, hauling whatever treasure they can carry and considering knocking out the torchbearer's knee in order to slow those trogs down.