This idea entails the same thing as the most basic premise of good composition: an irregular breakup of space is always more interesting than a regular or symmetrical division of space.
This is true of layouts and characters as well.
I think we have a naturally tendency to want to make things even and symmetrical. It's a challenge to train yourself to draw in a way that makes things uneven and interesting, but these elements in a design make it appealing to our brains, probably because it runs counter to our instinct for straightening things up and making them even.
Even in a simple drawing, just changing the proportions so they are uneven can add a lot of interest.
I got these pictures of the "Funny Face" characters from Dan Goodsell's site. It's got a TON of great designs from cereal boxes, etc. Check it out. He was nice enough to give me permission to use his stuff here.
These are so simple and so appealing. There's not much to them so you have to ask yourself "why do they work so well?" I think the proportions make them interesting; they are all a BIG fruit shape with little features pushed up towards the top of the face. Also the features all have a real nice rhythm to them. They are a good example of how much you can do with very few elements! Less is more.
If you check out Dan's site take a look at how the "Funny Face" characters evolved over time. Like most things that were designed in the 1960's, they were redesigned as time went on and by the 1970's they had lost what made them interesting in the first place!
When you design two characters that have to work (design-wise) with each other, it's good to give them proportions that are really different from each other so just looking at the two of them creates contrast and interest. Mike and Sulley are a good example; very different proportions create immediate visual excitement.
Actually Mike and Sulley are a good example of some other things we've talked about already; each has a good variety of:
Small, Medium and Large shapes
Type of Shape (Circle, Square, Triangle, etc.)
Any lineup or group of characters should differ from each other in their proportions as much as possible to create visual interest. When you know two characters are going to be in the same scenes together make them different in their proportions.
In a collection of different animals it's easy to get a lot of different proportions. With people it's a little harder but people come in all shapes and sizes as well. Also clothes can be used to give each character a totally different look and different proportions. We've all seen the old guy with pants up to his chest.
So you would think that it's easy to make two characters interesting; just make one huge and one tiny, which will create contrast and be interesting.
You could, and that would work, but when you think about designing characters for animation purposes, one of the things you realize right away is that all the characters you design have to have the ability to interact with each other just like real actors in a live-action film do. They need to talk to each other, communicate with each other and sometimes they need to be able to have scenes of great emotional weight with each other. So you'll find that if you design a huge character and a tiny one it can be hard for them to interact. It can create problems in staging scenes between the two characters if one has his head a mile in the air and the other one is down in the dirt. Eye contact between characters is essential to their interaction and being able to express themselves to each other. Mike and Sulley are actually pretty unique for characters in feature animation because they are pretty different in height (other than Tinkerbell and Peter Pan, or Pinocchio and Jiminy, of course). I'm sure if you watch any of these three movies you'll see that the filmmakers came up with a lot of interesting solutions with their staging to get the characters face-to-face in the same frame when they needed to communicate with each other.
Proportions are very important and they can be very subtle. Mickey Mouse is a very simple design, and yet it's hard to draw him so he looks right. His proportions are tricky to get in the right relationship to each other.
We've all copied a drawing or even traced over a drawing and it didn't look quite right. Why is this? Because we made a subtle mistake in the proportions that threw off the whole drawing.
Being sensitive to proportions - especially subtle ones - is hard. It takes some time and work to train yourself to see proportions correctcly. But they can make the difference between a good drawing and a bad one.