It would be absolutely impossible for any footballer (soccer to the yanks) to put on any substantial size with all that amount of running/training these guys do.
When was last time you saw muscular footballer?
They probably burn like 5 thousand calories a day...lol
It depends on how you define "muscular". Carrying too much size is obviously a hindrance because it can make you cumbersome and can affect your endurance levels. However, being muscular but in a slim and athletic way is perfect. Players like Ronaldo, Bale and Drogba have the ideal physique for a footballer.
Of course, it depends on the position you play too. Goalkeepers for instance can afford to carry more mass since height/reach are an major advantage and less endurance is required.
Ex-goalkeeper Stuart Tomlinson had probably the best physique in British football (soccer):
