Usually captive bred animals are smaller than their wild counter parts. This reduction in size is the "domestication effect," which is through captive breeding (i.e. domestication) an animal has certain attributes reduced in size - especially if they are a much larger in the wild. Now I'm not saying that these frogs are being domesticated (because they aren't), but the effect of size reduction is similar. There are many envrionmental and diet related factors that influence this "domestication effect." Sorry not to nerd everyone out :-)