I'm not saying height is everything, but it certainly helps...
he cannot stand his ground next to a strong striker in crosses or corners, his heading is average
he's a beast at sliding tackle, commanding the line, bravery etc but physically he is held back
players like Puyol (i reference him a lot because i'm sure most of you saw him play in his prime) were not pushed and out muscled easily, our biggest weaknesses came from crosses / corner
I see mascherano as a better version (skill wise / defensively) of prime Pepe (whom i rate highly despite his anger issues and antics) but Pepe had height/physicality which helped him a lot