I never did math past linear algebra/real analysis, so the only concept of sizes I have are countable/uncountable infinities.

Apparently the crux of this proof was showing that "the space of all modular forms with bounded denominators" and "the space of all congruence modular forms" were the same size.

I wonder what kind of expression "size" is here. Presumably not some finite integer, nor one of the simple infinities, since their first step was showing one is "a bit bigger" than the other. I wish this article went into more detail on that.

I definitely remember nerding out about modular forms via Andrew Wiles as a younger self.