Size is part of it and some guys even at the same size are more injury prone.
Part of it is genetics and part of it is composition and part of it is how they take/avoid hits.
I haven't followed Tua as closely as some here. But the times i watched him in college he seemed to be hobbling/limping every other play after a sack or some form of contact.
Yet you watch guys like Russel take sack after sack and they seem to walk away without a hitch.
Then there are guys like Murray that seem to never take a shot.
The list goes on and on for guys of comparative sizes.