I have no doubt that stag can walk, run and carry those antlers just fine... If they are 20KG in wieght and a normal big stag carrys 10kg it is not unreasonable to put double the weight onto his head (slowly as the antler grows) and he will develop the strength to carry it.

This is genetic manipulation - selective breeding at work.

Cingale - the SCI record books separate these animals from free range animals already... correctly so!!