I know how good Yaya Toure can be, but he is slightly lazy. While he is very good on the dribble, Mascherano is very good playing one touch football and his ability to stick right in front of the back four is quite commendable as players these days are very keen on going forward and leaving spaces behind.
You'd be surprises as to how much space yaya leaves behind, he is first and foremost a CM. Mascherano has the ability to read plays and intercept passes where as Yaya is very good at one on ones. He is more of a creator than an interceptor.
Yaya has been working out for you because of his size, agility and his flair (for a DM) but more importantly, like I said, because Barcelona keep the ball for a long periods of time.