The "eat all you want" mantra needs to be kept in context as Jeff would say. The idea is really simple.
First understand a calorie is a calorie no matter where it comes from. It doesn't matter if its a Calorie from fat or protein or carbohydrate. Its all a measure of energy and all calories are equivalent in measure.
Whether you gain, lose or maintain weight its all about:
Calories in versus Calories out
That's the magic equation and there's no more complexity than that to it.
The second point to understand is that foods have differing calorie density. I.e., on a per weight basis (really mass would a better measure but that's another discussion) foods have differing amounts of calories. For instance potatoes are only about 400 calories in a pound, while oil has about 3500 calories in a pound. So oil is almost 9 times denser than potatoes. Thus the focus on foods that have a lower calorie density.
The third point is satiety...feeling full. Certain foods keep you feeling fuller both quicker and longer. The sooner you feel full the sooner you will stop eating and so eat less. The longer you feel full the more likely you'll either eat less frequently or eat a smaller meal next time. Starches such as potatoes tend to make you feel fuller quicker and for a longer period of time than other foods. Proteins and fat do not keep you as full or for as long and will generally be found in foods with higher calorie density.
The fourth point is about eating to be satisfied. Not eating to be absolutely stuffed till you can't walk anymore and fall asleep. Eat till you are no longer hungry and feel like your sufficiently satisfied you aren't hungry any more. This normal feeling of being satisfied can take some time to reach as you must remember your poor eating habits over years has really screwed up your normal satiety/hunger mechanism. It may take some time, a few weeks to a few months, before you get full control back in your hunger levels. But be assured they will come back to normal levels. You just need to stick with it. If it means you at first eat a bit more than normal, that's OK. Over time your body will adjust and you'll soon be seeing your hunger levels return to normal, healthy levels. And yes some people are volume eaters and some eat smaller portions throughout the day. It doesn't matter what your comfortable with in your eating style, do what you feel is best for you. The McDougall program easily fits both styles/patterns of eating. Ultimately, making the distinction unimportant.
The fifth point to understand is this is a lifestyle change. It is NOT a diet. The program is meant to make you as healthy as you can be for life and that means you must incorporate it as part of your everyday life. Sure you can use it to simply lose weight and gain some health back, but why stay on that roller coaster! Once you start and embrace this lifestyle you will soon wonder why you thought it was so difficult to stick with. Your increased health will be all the reason you need to stay with it for life.
So finally, a combination of these five points together is the proper context to think about when hearing the "eat all you want" mantra. Its about eating till satisfied by eating lower calorie density foods that just happen to be primarily starches and vegetable/fruit based with a little bit of nuts/seeds/higher fat foods once you reach your weight goals.
One last point, those foods that you do eat should be as unprocessed and in their natural state as you can find. Processing almost always increases calorie density and makes food less satiating so you end up eating more calories. If you don't need to lose much weight then this may be ok for you, thus the regular McDougall plan allows some processed foods like flours, pasta and bread. The MWL program is stricter and doesn't allow for processed foods (other than tortillas).
On either the regular or MWL programs you will lose weight, you will just do it quicker on the MWL. But the potential downside to MWL is that it is more difficult to stick with for some and thus may discourage you.
If you have a lot to lose the regular plan will work quite well for you. If you are close to your weight goal the MWL may make it easier to lose those last few pounds. And if your really motivated and want to lose as quick as you can then try the MWL. If you find it too hard, then just simply move to the regular program by adding in a few processed foods that you might like. Either way you will lose weight and become significantly healthier in the process. Myself, I find that if you can stick with MWL for a month or so you will find it becomes relatively easy to stay with. But on either program you will lose weight over time and put yourself in the best health you've never had.
So I've written too much now and bored everyone so here's the final summary points for those that want to cut to the chase:
1. A Calorie is a Calorie.
2. Calorie density is key to reduced calorie consumption. Starches (grains and tubers), beans, fruits and veggies are all low in calorie density.
3. Control your hunger with lower Calorie dense foods that fill you quicker and last for longer periods of time -- i.e., starches
4. Eat until satisfied, not till your stuffed! And it doesn't matter if you eat a few big meals or a bunch of smaller meals throughout the day.
5. This is a lifestyle, not a temporary diet! Embrace it as the best way to fully regain your health for the rest of your life.