> My theory is self storage is actually in competition with getting a bigger house.

Obviously true. If one had a house with space for posessions, there would be no need for storage. As houses become ever smaller (for a given $/sqft) external self-storage becomes the only affordable way to gain some space.

My parents were poor but back ~70 years ago it was still achievable for a lower income family to buy a house with some space. As an engineer in Silicon Valley, the largest house I can afford is barely twice the square footage of my childhood bedroom. In such a tiny house we fit 3 people, all with hobbies and equipment, child has toys, etc. A house with enough space would be another million at the very least. So that's impossible.

So compared to that, a couple hundred a month for self-storage is a bargain. I hate it, would rather have the equity in a larger property but it is impossible to afford.

So yes, that's why storage facilities have such a boom. Housing by $/sqft is impossibly expensive.