Duquesne and Tourville emerged from the unsettled cruiser hierarchy inherited from the First World War. The Washington Treaty imposed a 10,000-ton cruiser ceiling, yet designers were still thinking in terms of scout cruisers, armoured cruisers, and much larger battlecruisers. An eight-inch gun did not automatically make a ship a purpose-built heavy cruiser in that older framework. Their later heavy-cruiser label can obscure the lighter scouting assumptions built into the design.
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Why Duquesne began as a light-cruiser design
The Duquesne class carried eight-inch guns but was conceived in the older light-cruiser framework before treaty categories hardened.