That does seem small for a BI. He looks healthy with a nice firm muscular body. He doesn’t look underfed, but maybe the feeding style may have been “maintenance” style inadvertently. Just be sure the food item matches his girth or a bit less. If his size is in between then I feed 2 smaller prey items every other feeding before moving up to a size that is too big right now and wait until he gets bigger.
But, like any species of an animal, he could just be a runt or late bloomer. I would not be surprised if Anubis has a growth surge these next few years.