For any group having a group size higher than 10 contacts, you will be able to split the group into multiple smaller groups.
To do that, just swipe right on a group name in your groups list, you will then see the option allowing you to split your group into smaller groups.
You can see this quick action in the below figure.
After selecting this option. You will be requested to enter the number of contacts you want per splitted group.
The resulting smaller groups will have the same name as the initially splitted group with a sequence number and will be immediately available in your groups list.