I think they probably did get really big. There are some cases of dead sperm whales washing up that have appeared to be attacked by giant squid. Coupled with the fact that the oceans used to be teeming with life before commercial fishing I would imagine perfect availability would definitely get you some big squid.

This paper talks about fishery populations during WW2 when it was to dangerous to fish. So if you think about how many fish would be available, I imagine that you could have many giant ones roaming around. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2929348/