Uhm, why can‘t a half breed just have the physique of her mom, though?
Not how genes work in the One Piece universe.
If a human has a child with a Fishman or Merfolk, the child will still have some of those features.
You're thinking of the Buccaneer genetics as being traits like eye color. Humans can have dominant or recessive traits with eye color, so, people with brown eyes are generally going to produce children with brown eyes when mating with people with blue eyes. But that's between humans of the same race.
As far as we know in the One Piece universe from Fishmen/Human hybrids, the racial elements are dominant traits, meaning Bonney would have some Buccaneer attributes. Kuma was a child between a Buccaneer and human, and had full Buccaneer traits.
This is why Big Mom wanted to marry into the Giant race: if Lola and Loki had produced children together, they would have had Giant blood and given her a great army (same thing the Marines want).
If Kuma was just a big human dude, it would make sense for Bonney to be his daughter and look like a normal human. Since he's from the Buccaneer race, some element of his race should be present in her biology, and there isn't any we know of, unless the Buccaneers have special abilities beyond their large physical stature.
Going further, I believe the Buccaneers are human/Giant hybrids, and that we'll get more info on their race/tribe in Elbaf.