Tall and skinny (but athletic) means that you have more time to perform tricks meaning it's easier for you to perform harder tricks. Look at the top 10 from worlds this year compared with 5 years ago.
Of course this could just be coincidence, we'll have to wait and see, but I'm making the prediction now anyway. Probably 2009 is too short a time period, but by 2019 I'm predicting the majority of players in the top 10 at the main events at worlds will be tall and skinny.
Not that this doesn't mean you can't be short and fat or some other combination and not get good at footbag (although obviously not actually fat, just stocky). I'm certainly going to keep playing despite this observation