The size of a rear bag does not tell you how much a cargo bike can safely carry. In its guide for the second-generation GSD, Tern specifies a maximum gross vehicle weight of 200 kg. That figure includes the bicycle and battery, fitted accessories, rider, any passengers and the cargo itself. If an unbraked trailer is attached, its weight and contents count as well. The 200 kg figure is therefore a limit for the whole loaded vehicle, not an allowance for luggage alone.
The overall limit is only one part of the calculation: individual racks, panniers and platforms have their own lower capacities. Put heavier items low and towards the rear, where this model’s rear rack is designed for greater loads, and leave lighter items for the front. Keep the weight reasonably even on both sides and secure every item so it cannot shift or reach a wheel. Before setting off, check both the total loaded weight and the rating of each accessory in use.
A heavily loaded bike also feels different to ride. Tern recommends practising in a safe area first and allowing more time and space to stop at junctions. Brake earlier and more gently because the extra mass changes how the bike slows down. The same planning principles are useful on other cargo bicycles, but their exact vehicle and rack limits must come from their own manufacturers. A good loading plan starts with weight and balance, not merely with filling available bags.