Why treaty cruisers grew toward 10,000 tons

Why did navies move so quickly from smaller light cruisers toward the 10,000-ton treaty limit?

The larger hull could carry substantially more armour and firepower without spending all of the extra displacement on propulsion, making smaller cruisers poor competitors.

A 10,000-ton cruiser had much more weight available for protection and firepower than an existing 5,000- to 7,000-ton light cruiser. Its machinery did not need to consume the added displacement in the same proportion. A smaller cruiser would therefore enter the fight with a structural disadvantage even before particular gun layouts were compared. Treaty ceilings became practical targets because no navy wanted to concede that margin to another signatory.