Why did they get so big???

There is nothing in the film to explain what happened rationally. You have it right that there is a vague hint that something more MAY be known. But the cops may simply be dealing with some totally inexplicable stuff that has happened before. Really, nothing explains it.

By the time our protagonists were working their way down a corridor of spiders, I had decided, that absent any horseshit about strange chemical experiments or a mad doctor splicing genes, this all just has to be taken at face value and the supernatural becomes as reasonable as anything.

It seemed like they were getting bigger with each "generation". Maybe their natural size in the wild is the result of being birthed from insects and small lizards, whereas being birthed from humans over several generations, there's more nutrients to grow them to a bigger size.