I think the perceived size of one's forehead has a lot to do with the shape as well as the height of one's hairline.
Specifically, on what happens above the temple, in the place in which this model's hair is sparser than everywhere else (skin shines through). If there is hair there, the hairline is a more or less smooth curve from one ear to the other. If there is no hair there, the hairline is more or less angular - straight up from the ears, and then either straight across or with another angle (a "widow's peak") in the centre. If this model were to shave those places, it would make her forehead considerably more prominent, even though the actual increase in area is only on the order of a few square centimetres.
It's my impression that the single-curve hairline is found more frequently among blacks than whites, which would explain the OP's generalized perception. Needless to say, there are plenty of counter-examples, though. Which is not to say that there may not be other contributions: Actual differences in forehead-shape, or wider cheekbones which make foreheads look smaller in comparison, or more voluminous hair which again make a forehead look smaller, et cetera. And there may be ethnic tendencies either way, in each case. Lastly, even skin- and hair-colour itself can likely influence size-perception, to some extent.