Got carried away there laying out the required patchwork of theories, but as regards giants and the such... The water from the Hydroplate Theory event is suggested to have had a higher quantity of Deuterium in it, which could've slowly corrupted / affected the telomerase, shortening lifespan, shrinking humans, etc.

An other possibly relevant theory which I haven't yet gone deep enough into yet is Archaix's "vapor canopy" theory where the atmosphere completely shifts into some higher oxygen composition, meaning metabolism changes and possibly giants form from the young experiencing completely different air, food, light, etc over generations.

Eh food for thought. The vapor canopy idea is still "cataclysmic" so it's worth hiding by the elites, I'd guess.