Here's an article on why being shorter helped out in the pros for attacking players...it has to do with the steps being shorter and being able to turn quicker.....
Unlike in other sports, some of soccer's biggest stars are small in stature.
www.theatlantic.com
It's also true, however, that among the male pros in Europe, the players have been getting taller, including strikers
European footballers are on average playing at the top level until they are older, growing taller and running faster, according to a study of football across the continent published on Wednesday.
www.reuters.com
www.theversed.com
I think the most likely explanation is that the increase coincides with the rise in the academy system, and to survive the academy (given how ruthless it's become and how few are now willing to take a chance like on a short Messi) the player needs to survive year over year. Among the boys, something happens around age 12 (earlier for higher flight players, later for lower flight players)...the GKs begin to get their training basics down and the defenders begin to master the fundamentals of the offside rule so it becomes a lot harder for players to score just from a direct shot on goal. That means you are left with scoring on a PK or kick, a 1 v 1 (or higher like a 2v1) or a cross. For the cross you have to be able to beat the goalkeeper or defenders to the header (most European countries don't have the antiheader rule for the very young)....if the GKs and defenders have been increasing in height, the strikers must as well to be able to get the ball on the cross.