I remember when games where a couple of MBs.

It's a natural progression, of which the answer to the question is advancements in graphic/photo quality, realistic open world environments. I remember oh 20-30 years years ago where I looked at a single really high quality photo image, and I can't tell you how big it was, but for the time it was HUGE and this was just one image. I want to say it was in the 2-10GB range when my hard drive was something like 500MB. And 30 years before that computers took up entire rooms. 😲