Do you need to get stronger to get bigger? The short answer is, absolutely you do.
The more complicated answer is that there will come a time when you will need to incorporate other tactics to stimulate further muscle gains, because you will approach your absolute strength limit long before you hit your absolute limit as far as how much muscle mass you are capable of building.
In the beginning, there is almost an identical pattern of strength and size gains. Weight training is such a new type of stress to your body that it struggles mightily to adapt. This adaptation comes in the form of increased strength and muscle mass. Beginners often see very decent gains even when they fail to eat enough or get enough sleep, mainly because the stress is still such a novel thing.
But as time goes by and you become stronger, you need to keep using more and more weight. You can't just do more reps with the same weight, because after a certain point all you are building is muscular endurance, which is not the same thing as strength. This is why, for example, just doing push-ups with your own bodyweight will give you some results in terms of size in the chest, shoulders, and triceps up to a point. Once you are able to knock out 30, 40, 50 or more push-ups however, you will find that you are doing more and more push-ups than ever before, yet your muscles aren't growing any larger. This is when most guys realize that they do have to switch to a bench press in order to place heavier loads, since their own bodyweight is now far too light to stress the chest, shoulders, and triceps.
This basic concept can be applied to all the muscle groups. If you start off struggling to squat the 45-pound Olympic bar for 10 reps today and in a couple years you are doing the same 10 reps with 315 pounds, there is no question that your legs will be far thicker and more massive.
But now another concept needs to be understood - TUT, or time under tension. Why is it that we advise 8-10 reps for the upper body and a bit more for the lower body, instead of simply doing maximum lifts? Not all the mechanisms of muscle growth are understood yet, but one thing agreed on is that if the muscle isn't kept under tension for long enough, you will see gains in strength without corresponding gains in muscle mass. Many of you have probably watched Olympic weightlifting and witnessed seemingly impossible feats of power, especially in the lower weight classes. For example, the world record clean and jerk in the 56 kg. weight class (123 pounds) was set in 2001 by Halil Mutlu of Turkey, at 369.6 pounds. How can a man that small be so strong, and why isn't he heavier and more massive? It's because training for explosive power is not the type of training that stimulates muscle growth. Training for power is really more about training the nervous system and the connective tissues rather than the actual muscles.
I see a perfect example of this every day at home with my wife Janet. Janet trained with me for many years as a bodybuilder before switching over to CrossFit in the summer of 2010. CrossFit has plenty of weightlifting in it, but the goal is always expressed in terms of maximum lifts or as many reps as possible. They are never concerned with the feeling in a muscle, getting a pump, etc. Janet is technically stronger now than she ever has been. The other day she pulled a new personal best deadlift of 300 pounds at a bodyweight of 134. Just a couple years ago, she was about 150 pounds and carried far more muscle mass - because she trained for size and did 'sets' of each exercise that kept the target muscle under tension the whole time, rather than a series of explosive efforts as she does now.
Another thing to consider that I alluded to earlier was that eventually there will come a time when you won't be getting much stronger, if at all. If this were not true, there would be plenty of guys bench pressing 1,000 pounds for reps and doing the same with 2,000-pound squats. Luckily, you can continue to stimulate muscle growth even though your strength is maxed out. Ways to do that include focusing more on contractions, slowing down the rep speed, super sets, drop sets, forced reps, and pre-exhaust. You can also try new exercises or older ones with a slightly different angle of pushing or pulling, different grips, hand width spacing, foot stances, etc. Even something so simple as changing the order you do your exercises in or shortening your rest times between sets can be enough of a difference to force the muscle to keep adapting and growing.
As the years go by, it becomes increasingly difficult to make any further gains in size or strength, simply because you are approaching your absolute limit (injuries often also play a role as well as more nagging issues like tendonitis). What is your limit? No one can answer that question for you or even tell you how long it will take you to get there. Some people reach theirs after only 4-5 years of training, others might keep making slow and steady gains in size, strength, or both even after 20 or more years of training.
This brief discussion has barely scratched the surface of the subject, but hopefully it's enough to get you thinking and looking at things in a slightly different light.
Talk to you all later!