I have no personal experience with weight watchers, but I think for a lot of people, it's a good plan. Understanding what you eat and how much you eat, is a big factor in losing/maintaining weight.
In this day and age, it's tough not to put on the pounds. Food is available everywhere, 24 hours a day. More and more of our eating takes place outside the home, and the portions we're served, tend to be too large and densely packed with calories.
Good luck with weight watchers. I'm sure that over time, you'll simply take on a new way of eating, and not have to struggle every day of your life. Your new way of eating will simply become the way you eat.
The smaller size is just like the large size, sooner or later there is no more left.