As this is my first post, I preface it with the fact I am 48 and have been bodybuilding since 1978 - started at 15.
I tried both types of workouts for many years each - volume (warmups followed by several sets half-pyramid style) with several exercises per body part, and "high intensity" (although NOT the Mike Mentzer "extreme" where you work out one set per body part for once a week or sometimes even less).
I made my BEST progress on EVERYTHING (size, endurance, strength) by using a rep-range of 8-12 on average, ALWAYS going go failure on final set for each exercise (which was usually one set per exercise), used VERY STRICT form, and did a half-body split alternating days with 2 days off on weekends. Don't know if any of you remember his name - it escapes me right now: he used to ride a bike to the gym in the early 80's with a basket on the front
When I went to absolute failure on each set, it was not only my muscles that were being taxed, but also all "support systems" (hormonal, such as cortisol - which you want to try to stop before "exciting" it too much, HGH, adrenal, CNS, etc.). Your body simply cannot take the stress of a 2 hour a day workout every day, 6 days a week. By experience and watching others for 34 years, I saw NO difference in non-steroid users between those who worked out 2+ hours a day for 6 days a week vs. those who worked out 3 days a week for no more than 1 hour a day. Conclusion: (and yes, I realize it was not a double-blind study that should be used to render a statistically defensible conclusion) why work out as much as 5 times more than you need to accomplish the same goal, unless you just like to hang around a gym?
Caveats:
1. Someone asked if steroids played a role. The answer is obviously yes. They enable people to stay in the gym longer and return sooner because they are enabling your body to recover much faster than the non-steroid user (unless you are a true genetic anomaly like Flex Wheeler, whose body was tested and was shown to have a myostatin/cortisol gene "defect"; even Arnold said Wheeler was the best bodybuilder he had ever seen).
2. Genetics are ALWAYS going to trump the non-gifted person, no matter how hard the non-gifted person works out. I am NOT saying "don't work out - you don't have a chance", but why do people view bodybuilding differently than they do any other professional sports? Guys, people become professional athletes BECAUSE they are genetically gifted in their chosen field/profession. How many people do you know as small as former pro basketball player Spud Webb being able to regularly dunk a 10 foot basketball goal??? How many people do you know who could run like Jim Brown or guard a line like Dick Butkis? The weightlifting sports are no different.
3. Finally, about cardio. I have no interest in Arthur Jones's former businesses (Nautilus, which became MedX) nor his son's, so what I say is not impacted by either of them. But Arthur said a long time ago one thing that always stuck with me, and I asked several professors (all PhD's in the "body field" - kinesiologists, bio-mechanical engineers, sports medicine MD's) about it and every one agreed: when you work your body intensely so that your raise your heart beat to a certain level and keep it at that level for a set amount of time,
your body is basically "doing cardio" because your brain has no idea WHAT is causing the increase in heartbeat and length of that increase!!! Most people think that "cardio" must involve training with something OTHER than weights - nonsense! If you start working out your legs doing squats with a huge load and are puffing and blowing like a freightrain, and continue training your legs with very little rests in between sets until you are finished, do you think your brain registered "Oh - we were just doing bodybuilding leg workouts, so that doesn't count for 'cardio'"? Of course not. Your brain registers an intensity overload that increases heart rate and length of elevated heart rate, REGARDLESS of what is causing that heart rate.
P.S. - another instructive thing to just view/look at the bodies of runners at not only the recent Olympics, but all running sports: look at the bodies of sprinters vs. long distance runners. Why do sprinters look like bodybuilders and sprinters look like Lance Armstrong? Usain Bolt is a true genetic anomaly because of his body's ability to fire impulses to his legs like no other man seen before AND, something that many people forget because they marvel at his speed - Bolt's 6-5 frame enables him to cover the same distance (100 meters) taking 44-45 steps while all other runners take up to 49-50 steps. It does not take a study to figure out that if someone can fire his synapses just as quickly as other runners but has to take less steps to cover the same distance, that person will more often than not beat the other runners, all other things being equal.