Purebred Brahman cows' system does not allow a calf to grow beyond a certain weight - no matter what monster bull she was bred to, BUT, the next cross-bred generation is a "free for all". She genetically may be carrying genes for huge birthweights from the "monster sire", and she can pass those genes to her offspring.
Simmental/brahman breeders that were creating the Simbrah breed ran into this problem.
So, just because they are F1 brahman and they themselves were low birth weight cattle, I would still be very cautious as to what I bred them to. I would want to know more about the other half of the breed genetics.