I know I’m going to get flamed for this, because I don’t even believe it myself, but a lot of these really huge creatures would make more sense if gravity was less in the far distant past.
Gravity wasn’t weaker, but the atmosphere was thicker.
http://www.bearfabrique.org/Catastrophism/sauropods/biganims.html
Megafauna and the attenuated gravity of the antique system.