Why cruiser growth stops at the fleet's support capacity

How large would cruisers become if 15,500- to 18,000-ton ships were normal?

Probably not much larger, because servicing capacity and the relationship to battleship size would impose practical limits.

Cruisers already in the 15,500- to 18,000-ton range would probably not grow much larger in the proposed setting. Bigger ships risk exceeding the navy's servicing capacity. They could also outgrow its battleships, making the distinction between the types awkward. The armored-cruiser era is acknowledged as an exception, when some cruisers were larger than battleships.