Looks like a pretty hefty frog to me!!! I see the concavity that you mention, and that is slightly odd. . . but the generic term 'African Dwarf Frog' refers to one of several different species -Hymenochirus boettgeri and Hymenochirus curtipes are the two I can think of offhand - it's possible that your frogs are a different type than the ones I've kept . . .
If you aren't seeing any pimples, then you have girls, and compared to skinny boy frogs, the females can be bruisers! They're ALWAYS huge compared to males, lol. They're also very greedy at feeding time compared to the males I've kept. . . from the images posted, they look okay to me, definitely big girls, but healthy. Your feeding schedule is good for adult frogs, too. I'd vary the wetfrozen foods if possible, but since you're feeding a quality pelleted food, you're probably okay. . .
The stomach of ADF can easily be seen if you look at their underside, it's in the middle, but slightly to the left of their spine. You should be able to tell if your frog is getting enough/too much/or not enough by taking a look on their left side before/after feeding. A hungry frog will have a bit of flatness or even concavity (just a BIT!) to the left side, where if the frog is full, there should be a slight bulge on that side. If the belly bulge is REALLY bulgy, you're feeding too much, and should cut down a bit.
Really, in my experience, these little babies come in a fairly wide variety of shapes and sizes, genetics definitely plays a role, as all of mine have been very easily distinguished from the others by both physical characteristics as well as behavior. As long as they're behaving as you have come to expect from them, with no odd sudden changes in feeding, activity, or hanging at the top, you should be okay :)
Hope this helps?
If you want to get more opinions, you can always post your picture up here
. It's a thread on ADF that many of us keepers hang out on and babble about our undying love for all things froggy :) You're welcome to join in, and I'm guessing that more people will see and respond to your question over there!