ELI5 Why are Giant house spiders so big?

[u/Electronic-Volume-56]

I am complete and utter arachnophobe and in my country we are heading into the time of year know as spider season when all those massive house spiders get horny and run around looking for a mate and scaring me senseless. After getting my neighbour to save me from a particularly huge giant house spider I started thinking- why are they so big? They have been living in our houses for so many generations now, surely evolution should have selected smaller spiders that can better hide in our homes and avoid getting caught/squished/eaten by pets, rather than these hairy behemoths!