Ronaldo is by and large a more developed and polished player now, than when he was at Man U. Also as Big Mag said, at the beginning of his career at Man U, he wasn't the goal-scoring type of player that he was towards the end of his Man U career (and as he is at Madrid)....
I don't know if scoring in Spain is easier on average than in England. I think individual tallies in general at the end of the season are comparable between the two leagues. With the recent dominance of Madrid and Barca in Spain though, being the go-to goal scorer on any one of those teams is more of a blessing than it used to be... So maybe scoring at Barca or Madrid is easier than scoring in England (and in the rest of Spain)