But it hasn't be proved otherwise, has it? So I'd say we have to stay agnostics about it. It sounds only logical that height is somehow affects heading play. For example, if a Striker compete with Defender for aerial ball, then height might play a role along with skills and random factor.
Same goes to weight; we have Strength skill, maybe weight difference between competing players gives one of them a small bonus? It might be just a few percents, and this bonus simply stays unnoticed.
There is only one way to get to the truth -- we need to find correlation, which means gathering a ****load of statistics. Different players, different teams, different games... Alas, it's practically impossible.
Of course, I don't know for sure, but it's too bold to call it a myth without any proof, don't you agree?