It always amuses me when people equate strength with mass, and then go on to assume that if you're stronger, you're slower. As an avid fitness buff, I can guarantee you that's not the case, provided you train well. I'm only 5'7, and while my average weight is around 185 or so, during peak physical activity, I quickly add around 20 pounds. Do I look much bulkier? Not really. Am I more powerful? Absolutely. Ironically, because I build strength through dynamic strength training, with a heavy emphasis on isokinetics and plyometrics, I actually become much faster and more fluid as well. It's all about explosive, full-body movements, whereby I also increase my flexibility, quickness, and agility in addition to my power. By training properly, I'm able to recruit more muscle fibres to become fast-twitch muscle, which enables sudden, explosive movement.
People have to get it out of their heads that NFL players do nothing but pump iron. Nutrition and exercise have become sciences, with a focus on bio-mechanics and a keen understanding of what is holistically healthy. Old methods are falling out of favor, and new, safe, effective methods are taking their place.
I do parkour, gymnastics, capoeira, modern dance, physical theatre, swimming, crossfit, bouldering, slacklining, and a variety of other disciplines in order to improve all areas of my physicality. I'll never "max out" because I can always improve something. While I'm not a professional athlete, I am far, far more fit than the average weekend workout warrior. Yet I am not even close to hitting my full potential, and I look forward to the continual improvements I'll make. Besides, men don't fully mature, physically, until their mid-to-late twenties, so I still have a few more years before my body is even fully formed. The same is true for NFL athletes, particularly rookies.