Because muscle has a contractile and a non-contractile part. The non-contractile part (sarcoplasmic) is highly developed by higher rep training and adds a lot of size in bbers. Other atlethes lack this development of the sarcoplasm. Also, lower reps make your nervous system more efficient, and this contributes to strength. Well, sumthin like that. And no. To get big(but not necessarily strong) you don't have to lift enormous amounts of weight...
It isn't that simple, however. Many pro bodybuilders have also been or still are powerlifters. The original poster seems to think that he will get stronger without gaining much size. It needs to be understood how important food intake is. If you are eating to gain while training to get stronger, you will get bigger unless there is something wrong with your training.
Many powerlifters don't train for size due to weight classes. because of that, they may not eat enough to actually gain much weight. Strength isn't JUST related to muscle size as some strength is achieved through neural adaptation (your brain learning to do the exercise efficiently), and also technique. This will only get you so far, however. If those same powerlifters NEVER gain any extra muscle, there will be a limit to how much strength they gain in this area. This is one reason why many of the true heavyweight record holders are equally huge. They gain weight for leverage mostly but they are also carrying a good deal of muscle mass.
Bodybuilding, on the other hand, is strictly about size (even though more and more are turning to making the strength component equally important. My goal is strength and size. Weighing over 250lbs to me would be wasteful if I couldn't bench over 400lbs. That means the average bodybuilder will eat to gain weight along with lifting to gain strength. No one ever got 20" arms from curling a 20lbs dumbbell. Regardless of what you hear, it does take above average strength to get big unless you somehow relied completely on drugs to make up your size gain. I personally don't know any large bodybuilders who are weaker than average. I know a few who don't lift that heavy ANYMORE. However, that is a HUGE difference than never being strong in the first place. From older lifters I've talked to, it is easier to MAINTAIN muscle gained with a lighter weight over the age of 35, but it isn't like they didn't lift damn heavy to gain it in the first place. This is why you can't judge what it took for someone to get as big as they are simply from what they are lifting right now.