The larger Illustrious that treaty limits constrained

Could the Illustrious class have been made larger as treaty limits collapsed?

Yes. British planning considered a larger armoured-deck design with more hangar and aircraft capacity.

Reginald Henderson and the design staff pressed for options larger than the eventual Illustrious class, but political limits held the programme down. One alternative kept the armoured flight deck while adding roughly 3,500 tons, larger lifts, a somewhat taller hangar, and more hull length. Its air group would have four squadrons of about sixteen aircraft instead of three squadrons of twelve. Fighters, dive-bombing Skuas, and two Swordfish or Albacore squadrons could therefore coexist with the protected hangar. The result would be a more capacious development toward Indomitable within the armoured-carrier concept.