I think reducing the size of the stomach from a melon to an egg is the whole point of the operation, so that the stomach is able to tolerate a lesser intake, therefore reducing consumption and subsequently weight. The stomach still stretches with food, but because of its size, people will feel fuller quicker.
People who undergo the procedure are given counselling on diet and what foods and amounts are appropriate, but generally, of the volume of something is smaller, then it can only accommodate smaller amounts.
It is quite a serious operation with lots of psychological and physical changes. It is life changing and life saving, it is intended to be drastic. So when undergone, it would seem a little pointless to reduce it from a melon to say, an orange as the necessary impact would be lessened and weight loss take longer, leading to potential psychological failure through lack of achievement.