Why a three-tier cruiser treaty would squeeze out the middle class

How would naval treaties work with separate 10-inch, 8-inch, and 6-inch cruiser categories?

The system would probably favor large 10-inch and numerous 6-inch cruisers, with the 8-inch class struggling to justify its role between them.

The proposed treaty gives a 17,500-ton cruiser 10-inch guns, a 12,000-ton cruiser 8-inch guns, and a 10,000-ton cruiser 6-inch guns. Preserving space above the largest cruiser leads to suggested capital-ship limits of about 42,500 tons and carrier limits of 30,000 to 35,000 tons. Reducing the smallest cruiser to about 7,500 tons would separate the categories further, although that allowance is challenged as too restrictive. The 6-inch ship has a role in numbers and volume of fire, while the 10-inch ship offers greater fighting power. The 8-inch tier sits awkwardly between them. Reserving it for states excluded from the largest category supplies a possible political rationale, but France and Italy are considered unlikely to accept that unequal arrangement.