The human body has only a certain amount of fat cells, and they grow larger or smaller as we gain or lose weight, respectively. The number and distribution of our fat cells is set before we even reach adulthood. This is why we may have some stubborn areas where fat won't seem to budge, even though we are losing weight in other areas. This is where liposuction comes in.
When fat deposits are removed through liposuction, those fat cells are gone forever. The body's contour is improved, and ideally, the areas in questions are now more in proportion to the rest of the body. However, there will always be some remaining fat cells in the area, as well as in other areas of the body. Therefore, if the patient subsequently gains weight, the remaining fat cells will grow larger.
Since there are now fewer fat cells in the liposuctioned area, the idea is that in cases of weight gain, those areas will stay in better proportion to the rest of the body. However, be aware that it may appear that you are gaining more weight in areas where you did not have the procedure, since those areas now contain proportionately more fat cells.
In short, while it should improve your body proportions and contours, liposuction is not an effective method of weight loss, and it certainly does not grant you a guilt-free (or weight-gain free) all-you-can-eat license.