Looking at this shoulders and muscles for McGregor I cant think of anything else but him using some kind of steroids.
I do hope they will be testing them before the match.
It is very difficult to get away from USADA level testing as they might approach you anytime and if you run from them it will be another world wide news that Conor McGregor ran away when they approached them and i am sure he will not go that route, if you put on mass and start working out he will have an increase in mass and you really do not necessarily need steroids to make that change as he was not completely out of shape to reach this body mass.