It was probably not the space. Calhoun repeated the experiment multiple times, even with rats. We see rat populations that are much more crowded.

The "unnatural" part is socialistic care. Hence "Utopia" Those mice have everything material that they need 24/7. We, see such a population collapse already. Here in Germany, the human Utopia, the native population is on a decline for some decades. Japan, Portugal, Taiwan and many other rich industrial nations face the same problem. By 2016 1,8 Million Japanese people lived in self-chosen isolation (Hikikomori). People are already mentally ill.

Politicians think they can solve it by mass migration. And initially, you see a spike in birth, but after a few years, the birth rate among refugees and migrants declines.