Ok here's the deal, when will this confusion finally END!
Weightlifting heavy weights will not necessarily make you bulky.
Lifting light weights will do little to nothing for a pitcher.
Rep range is what determines hypertrophy (muscle growth)
A pitcher uses type II muscle fibers, these are explosive and powerful.
Light weight do not do anything for type II muscle fibers, they only work type I (slow twitch) muscle fibers, which are not needed for pitching. By adding weight they slow twitch muscles can no longer move the load on their own and one must also recruit Type II's to move to the weight, this is what we want.
Now a rep range of 8-12 is generally known as hypertrophy range, and this will add muscle size the quickest, although not the most possible strength.
Rep ranges of 6 and below add the most power and strength.
Now for the other statements,
Flexiblility actually increases with weightlifting through full range of motion and there are studies to prove it.
How many skinny pitchers are there in the major leagues, not many, and the ones that are generally stand around +6'5".
If you traing hard with becoming a better pitcher as your goal, lifting heavy loads and doing compound lifts, you will get stronger, and yes you body will probably look better. But, your goal shouldn't be 16" arms, rather, a strong and powerful body with which you can throw a baseball harder than you could before.
If you lift properly the chance of injury will NOT go up. In fact your flexibility will increase, and with a proper lifting program you will eliminate harmful muscular imbalances that create the oh so many hamstring pulls every season. Most hamstring pulls are from an imbalance of the quads and the hamstrings, a proper program will get your hamis stronger.
Yes Cantreras is a big guy, and yes he throws very hard, but I doubt it was from lots of bench pressing.
Key lifts for a ball player should be squats, deadlifts, power cleans, rows, lunges. I'm not saying benching is bad, I'm still undecided on that one.