I don’t know how to express it scientifically, but I will try to sound as clear as possible. Let’s say there is a ratio of the weight of the parent with that of the young it makes. In most reptiles, the weight of each young is minute compared to the parent’s weight, which is more pronounced in large species, extant or extinct. There are some small reptiles like some geckos and skinks with comparatively large young, but there are also many others, like small agamids, skinks and lacertids, with small young. In contrast birds and mammals produce relatively large young. Monotremes and marsupials give birth to altricial young, but when they start to get independent they don’t differ much in size from the precocial placentals of the same size. Even small placentals give birth to relatively heavy young. As a mammal gets larger, so its young. In birds the condition is even weirder, as even small species usually have less eggs and offspring than a reptile of the same size. And in larger species, the young get also larger, not smaller and more numerous, even in large terrestrial species with predators like ratites. Why there is such a discrepancy? Do birds and mammals suffer less from predation pressures? Does parental care play a role? And if it plays, then why birds with simple parental care like most precocial ground species still have less offspring, but crocodilians with parental care of a similar level have much more?

You really can't generalise in the way you are attempting to do by looking at one variable and then trying to identity a reason or a set of reasons why that variable occurs. There will hundreds if not thousands of different evolutionary pressures (good or bad) that will determine why the size of a particular animal's young varies between species. One can't say more than that.