Gastric reduction and bypass surgery reduces the capacity of the stomach to around 20mls, the equivalent of the size of a bottle of Tippex.
Before the operation, the stomach might be the size of a football or larger depending on the level of distension.
The surgery also involves a bypass of part of the intestine, with the first metre (which absorbs most of the nutrients) being bypassed. This means the stomach will empty into the intestine further down than normal.