Why a light cruiser need not approach heavy-cruiser size

Would a 15,000-ton heavy-cruiser limit force viable light cruisers up to 10,000 tons?

No; useful light cruisers could remain much smaller when their operational role did not require heavy-cruiser capabilities.

The Arethusa class provides an example of useful wartime light cruisers well below 10,000 tons. Penelope and Aurora are given as approximately 5,220 tons standard and 6,665 tons fully loaded. A larger permitted heavy cruiser does not automatically require a light cruiser to grow. The intended operations determine the size, and additional displacement has little value if it does not help the ship perform its role.