Have you tried mini mealworms the nice small ones,packed in freezer sections usually they thaw right out and if small enough they will go after them,just watch putting them in containers with towel or substrate tho as they will get lost and not eaten
Flies or fruitflies make up a natural diet that's easy for you to collect.
If you have dog poop available, that can attract a variety of different sized flies. You can also attract different sized fruit flies with a couple banana pieces, peels, or citrus rind.
How many are you feeding?
"Seeming" too small may just the way you are judging size difference. Try a couple with flies, and try a couple with your hydei. You'll see what works.