What I remember is there was an arms race between plants and herbivores. The plants tried getting bigger to get away with herbivores, so the herbivores got bigger, grew longer necks, etc. Since the herbivores were so much bigger, the predators had to get bigger too.
Grass evolved 30 something million years ago, so now herbivores don't have to feed high. That also means there's no point to herbivores getting gargantuan again.