because he's managed to get it down, don't be concerned about the size of it, you'll be surprised how much axies can eat.
I would however be concerned about their treat, pieces of liver and heart seem to be a favourite of treat for giving too axolotls, but they are in no way good for them nutritionally, and I don't mean in the same way as 'cake is bad for you' I mean in the same way that eating grass is.
A nice big wrigglie earthworm is a lovely treat for axolotls, and because yours are still at the cute tiny stage, you can also give them some live brine shrimp, live daphnia, some live bloodworm might be nice too, I say while they're still tiny, because when they're older they don't even seem to spot daphnia or brine shrimp.
when my ones were at the 2-3 inch size they were both most happy with a bag of live brine shrimp.