That's why the ones with the belts are progressively being withdrawn, and replaced with self-checkouts that have much larger bagging scales. They can easily take a trolley load of groceries in bags.
The ones with belts have not proved popular. In many Tesco stores, the staff are trained to encourage customers to use them; this is necessary because people would rather queue for the self-checkouts with bagging scales. They are also much harder for the staff to observe/supervise, which is important when trying to reduce fraud. Finally, they are less reliable because detritus accumulates in the belt mechanisms. The debris causes the weighing scales (under the first belt) to become inaccurate quite quickly, which means the checkout has to be taken out of service and a service engineer called.