I'm sure being fit is likely.
(I'm not a training expert)
But I'd have thought you build up muscle mass in preseason etc which can be caught up during the season.
I recall when I used to go to friendlies that the players always looked bigger due to lots of weight training in pre season.
Players who miss pre season due to injury or internationals do tend to struggle more the following season, but maybe that's just my mind fooling me.