Of course, it's common knowledge that the more you eat, the bigger you get whereas back in the dinos time, the primates diet was simply whatever's found in the trees which was not much & certainly not nutritious thereby keeping them small.

However, their small size was actually key to their survival as they were then difficult to see by the dinos especially in trees.

But anyhow, it took a few more million years of being on the ground (when they moved down to it since the dinos were already gone) that they eventually learned to stand erect since bi-pedalism [walking on 2 limbs] is a lot more advantageous than walking on four which the early primates originally did.