angel fish question

I would suspect that if all things are equal the small one could be a runt.

actually check to see if it's being outcompeted for food. I find this is usually the root of the problem.

I have a couple females that are noticably smaller than the same aged males. but it is more related to genetics as all of the smaller females are from the same brood stock and they are all marble koi. and they eat more than the males

it could be genetics, competition for food, or gender. my females are also smaller than the same aged males.

Angels can be finicky eaters. Try a variety of food.
This is what I would do.....feed your regular pellets that the other angels
like on one side of the tank and while they are eating, coax the other one over further from the others with some bloodworms and make sure he/she gets fed well.

I had that problem with wild scalare..I did the same..fed in one spot then coaxed the smaler fish over to another location.

while the wilds tend to prefer the frozen and live..I picked up the hikari brand bloodworms(they appear to be actual midge bloodworms)
I mixed flake in with it and fed them..they learned to eat the flakes this way.

keep an eye on the smaller one too..

I had one small one that was getting sick from being out competed for food..it actually eats out of a spoon now and is catching up to the others..I make sure it gets a good dose of bloodworms spiked with colorbits..it loves them..