Well for humans it makes a bit sense as humans breed with elves. Belfies/helfies does look like humans but with pointy ears.
But in case of trolls there is no history of that. And trolls have different beauty standars. It makes no sense for them to even look on frail and thin being when they prefer for their makes to be big and strong (and being able to actually survive the physical encounter).
Belfie bones would break if troll would just grab it strong enough (same applies to tauren) there is physical incompatibility.
Troll women have more meat on their bones, and since Zandalri are quite fixated on keeping their breed pure it would make more sense if they’d pick someone their size, especially when their women have the longest legs 
.