[bryanlarsen]

The fairings aren't constrained to the diameter of the booster, they already have a larger diameter than the booster.

The small size of the Falcon Heavy fairing is probably due to the fact that they are the same size as the Falcon fairing, and it was designed when Falcon could throw < 1/2 the mass it can currently throw, let alone the Falcon Heavy.

[throwaway48476]

Theoretically the fairing could be any size, but there is a limit to how much bigger than the rocket it can be.

[dotancohen]

I'm sure there's a limit, but it's not really that big an effect as you'd think. The fairing is lifted almost entirely by the first stage, and as SpaceX increased confidence in the landings they were able to reduce to fuel margins, leaving move for a heavier fairing. The aerodynamic effects are secondary to the added weight, and are only a really a bother for a few seconds at max q. In fact, the larger volume to surface area makes designing for max q easier in some respects, such as audio energy.