Washington Naval Treaty

I have no references to cite, but it's my impression that carriers were no more expensive when everything (air groups, operating costs, etc.) that made them operational was included than battleships. Carriers received the "best of everything" just like battleships.

The USN converted the old collier Jupiter to the first US carrier Langley. The Langley was an experimental test bed for aircraft carrier operations and operated as such until 1936, although in later years she was used mostly for training. The Lexington and Saratoga were converted from battle cruiser hulls which had never been completed, while carrier doctrine
(and carrier weapons) were still evolving. The Ranger was designed before much operational experience had been gained with the larger Lexington class carriers. This may have been part of the reason Ranger was considered a failure; it was not yet apparent that larger carriers had inherent advantages.

The British Colossus class carriers were a wartime class (1943-44) and were built to merchant ship scantlings, both to save time and cost. They were considered "light" carriers, but did not really have the capabilities of US light carriers, particularly in the area of protection and survivability. Originally 16 ships were planned, but only ten were built. The Essex class comprised 32 ships as ordered, but only 24 were completed. They were true fleet carriers and were the ultimate carriers of the WW II period.

There's nothing strange about it. The reason the converted carriers were allowed to be so large was that the vessels (Akagi/Kaga, Lexington/Saratoga) were all being converted from battle cruisers or battleships which were larger than any planned carriers. If the conversions had been limited to 23,000 tons, it wouldn't have been possible to use the already started hulls and these ships would have had to have been scrapped. That wouldn't have appealed to either the Japanese or American navies, both of which had their experimental carriers in operation before they commenced the conversion process on their capital ships. I think both navies felt the converted ships would be successful carriers, though, in the event, the Japanese ended up with major rebuilding projects in hand.