In 1922, an international treaty was signed that was intended to limit naval arms races. Among other stipulations, it limited treaties to a displacement of 10k tons and 8" guns. This limit was actually very high. Most cruisers before ww1 were much smaller than that, armed with 6" guns with relatively limited armor. But the british had just built a new class of ships almost that size 7.5 inch guns and didn't want to get rid of them, and the US Navy was generally favor of larger ships for its own reason, so the limit was set much higher than most existing cruisers.

Of course, once the limit was set, everyone immediately began designing cruisers right up to the limit, much to the consternation of the british who would spend the next decade and a half trying and failing to reduce it. As it turned out 10k tons was an awkward size. It was a little bit too small to make a really good 8" gun ship, and a bit too big for a 6" gun ship. This resulted in a lot of compromised 8" designs early on, most with too little armor or structural strength.

As time when on design got a lot better, for a lot of reasons. Power plants got more efficient, reducing weight and allowing smaller engine rooms with thicker armor. Designers also got a lot better at working within the treaty. the 10k weight limit excluded certain parts of the ship in the calculations, and designers learned to work within the legal restrictions. A 1922 ship and 1938 ship might both have a treaty weight of 10k tons, but in all navies the 1938 ship would have a true displacement considerably higher. As a result, the last generation of pre-war cruisers were usually substantial improvements over earlier designs.

The algerie in particular benefitted from a more compact machinery arrangement that allowed fewer boilers without losing much power, which saved a lot of weight and allowed the same weight of armor to cover a smaller area, meaning it could be thicker. The much of the hull was welded, not riveted, which also saved weight.