Why did Britain build two large carriers rather than more small ones?

Why did the Royal Navy not build smaller aircraft carriers and buy more of them?

The stated limit was two ships of whatever size the budget allowed. Smaller hulls would still have left the Navy with two carriers, so larger ships offered more capability when only one was available.

The stated purchasing limit covered two ships, with no option to exchange their tonnage for additional hulls. Smaller carriers would therefore still have left the Navy with only two. That restriction encouraged the largest ships obtainable within the budget, especially if only one would usually be available. Buying more small carriers would first have required permission to increase the number of ships.