Since your legs are the biggest muscle group on your body, should they be worked more than the rest of the body? And should the chest and shoulders be the most worked on upper body becuase they are the most major muscles there?
There is no reason to train them more often, because as Mårten says, the fact that your legs are bigger, ensures that you use more weight, thus training them more.
How often you train them depends entirely on your goals. For instance, a general worker-outer like me will do a split, and just rotate the days. But if you take a person who is actively looking to pull a record deadlift, he might have 3 leg days per week, and throw some chest days in on "off-days".
If you find yourself identifying more with the former group - a general fitness enthusiast - conventional methods will do just fine.
The general principle is that if you are looking to maximize your muscle mass, you work the larger muscles more than the smaller ones (i.e. more fatigue inducing reps). That means you would work your triceps more than your biceps, your quads and glutes more than your hamstrings, etc. You would also work your back and your general core more than everything else.
However, there are times where you do want to de-emphasize the larger muscles to focus on some of the smaller ones:
- You are out of proportion (i.e. your biceps are too small in comparison to the triceps)
- You are fixing a strength imbalance (i.e. you can't keep your shoulders in a neutral position because your chest is too strong)
- You are doing rehab exercises after an injury
The bottom line is that there are only guidelines, and even those guidelines can change depending on what your goal is. Sport performance is trained differently than general exercise, which has different emphasis than bodybuilding. Each sport has different demands, so you can't apply advice for a powerlifter to a soccer player (or footballer if you are from outside the USA).
Well, they already are. If you compare leg press with bench press, you'll probably leg press about 3 times as much weight as you bench. So the proportionality to muscle size is already there. Adding more reps or sets to your legs would be out of proportion.