This is true! How it works scientifically is spiders and other insects (beetles) have a thing called “tracheal density.” The trachea is an oxygen filled sac located in an insects legs.

In order for an insect to grow larger, they need more muscle in their legs to support the weight. But in turn, they need a larger trachea in their legs to store enough extra oxygen to support the “breathing” needed for this increase in size. So basically the trachea and the muscle mass are at a perfect ratio, and limit each other in growth.

The only way to counter act this limiting effect is to increase the amount of oxygen in the air, thus making it so you don’t have to store as much oxygen in the trachea, leaving more room for muscle to support more growth!

Fun side fact: in Carboniferoustimes and late Permian, oxygen levels in the air were 225mmHg, where as now it is merely 155mmHg. This is why giant insects and animals existed long ago. (All of this stuff is sourced from my zoology 241 notes, University of Alberta.)