Design Master Electrical uses the smallest standard breaker as the default breaker size for panels. Circuits with no load attached will have breakers this size.
To set the default breaker sizes, go to the “DM Electrical->Customization->Wire Sizing->Edit Project/Master List” command. In the bottom grid, there is the “Standard Breaker” column. The breaker sizes that are checked here are used when automatically sizing breakers. The smallest one is used as the default.