Cool..
I realize you have not had them very long, but do you know when they came out of the water? I have seen people sell babies all the from 2 weeks to 3 months old (and there is a considerable size difference there). So.. I am going to take the worst case and assume they are about 6 weeks old. IF this is the case, 1/4" may be to big.
Would I would do, get a crittercage of something similar, put a damp paper towel in there. Put the smaller frog, that you are worried about eating in the crittercage with a couple of the smallest crickets you have. Put the crittercage in a dark (preferably warm and damp) spot, and leave him be for a couple of hours. You should not do this until a couple of hours after you normally turn the lights out. Then.. count the crickets.. see if ate some.. if it did not.. it could be health issues.. or it could be that it just doesn't want to take food that is that big. Sometimes they get picky.. I feed my baby retf hydei fruit flies alot (they are the bigger of the fruit fly family) and pinhead or 1/8inch crickets.. See what you can find.. hopefully it is eating what you have and is just smaller than the other one..
If the skinny one eats in the critter cage.. put him in there only a couple of times a week.. let him try to fend for himself the rest of the time..
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