did you buy them when they where very little? has this frog grown at all? and is the other slightly bigger? may just be different sex. Female African Clawed frogs tend to be larger than males.
I had the first to as a pair they are about 2 inchs big now but when I but the green one in with tthem they were about an insh big the green one has only grow very slightly put abit of weight on but that's it realy. Thank you for the help