Latent genetics. If Hajrudin had gotten Harald's Oni genes, he would be bigger/stronger.
Loki hits the One Piece top-tier jackpot:
-Strong parent
-Strong area to grow up in
-OP Devil Fruit
-Busted genetics
You need A LOT to go right for you to become a top tier in this series. There are characters with crazy strength feats that would get one-tapped by the actual top tiers in One Piece. Hell, Hajrudin is one of them. Dude is VERY strong, and he's this much weaker than Loki.