Oh, and maybe I'm missing something, but why is being a large monster neccessarily a good thing? They can move a bit faster with the "large steps" between movements, and they are good at blocking passages, but they take up space and block line of sight for your own monsters and are huge targets who can rarely hide around corners. To top it all off, many of them (especially the ranged ones like nagas, elementals, manticores, and chaos beasts) are overall weaker than many of their smaller counterparts. I've only used large monsters a handful of times when given the choice, usually only for the fortitude of giants, and I have no trouble winning a majority of quests.
One of the bigger reasons is from the reinforcement rules from the base-game campaign (The Shadow Rune). When it comes to reinforcing, you can reinforce your entire monster group in a single turn (or half w/ 4 players) for many big monsters and a much smaller fraction of that with the smaller monsters. I've heard some of the other campaigns may have fixed some of these issues, but I can't point to a specific example yet.