Palm size would be about 3-4in from snout to vent, I think? That seems like a perfectly fine size for that age, at least my experience with my own Pacman had her in a similar range at that age.

Maybe some people's impressions are skewed because you can make them bigger in a shorter time span if you feed them a lot and the opposite could be true (feed less = smaller). As long as Jasper is healthy, eating, and all his other froggy habits are good, I wouldn't worry too much about his size.