I'm not sure myself. Whilst I don't doubt that it does happen, I don't think it's as easy as just saying that player x is too big and too fast, therefore must be on steroids.
Didn't we all have the kids in school who were 6'2"-6'4" by the age of 16 and still quick? There were a good 3-4 in my year at school. None of them were on illegal substances (well not the performance enhancing kind anyway

). The problem there was a lack of skill, possibly stemming from poor coaching, or just a lack of natural talent; and a lack of application (none of them trained particularly hard on their strength and speed).
There are always going to be a handful of guys like that in a country, who happen to combine size, strength and skill. Just a matter of statistics.