Thanks @Paul-Winterhalder and @David-St-Louis-0 . I'm communicating with our web dev and am passing on the information to him. I'll respond here once I get more information from him. Appreciate the quick replies.
The maximum # of group members is controllable by our support team.
Just send a request to our support team, indicating the name and appId of the app, and what you'd like the max to be and why. (A short description of your use cases would be helpful - since we just need to review to see if we think there would be any performance gotchas).
It was a bit confusing since I was thinking in terms of the owner having quick access to all their groups data, but I understand the difference in GetMyGroups now. It's a call on the profile and not on the group.