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 175 or so, during peak physical activity, I quickly add 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 explosive, full-body movements, I also increase my flexibility, quickness, agility, etc. 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, 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.
One last thought. Whether you like Ray Lewis or not (I'm not a big fan), it's inarguable that he's been a prolific and intimidating presence in the middle for many years. He's 6'1, 250, and as solidly built as any LBer I've seen. Yet, when he entered the league, he only weighed 230, and was considered an undersized overachiever. His coaches had him add about 5 pounds a year so his body would acclimate effectively, which means he took at least four years to get into the physical shape they expected from him. Funny how we're so impatient with our own players, expecting them to turn into superstars overnight.
Take it slow. Cultivate a long-term view of football--and of life. Trust me, it's much healthier.