There are reasons to make roundabouts with large radii. Ironically, some of them involve traffic-light controlled roundabouts, where traffic queued at a red light queues back around the roundabout. The consideration here is that one makes the circumference of the roundabout greater in order to provide more queueing room before the queue for one junction backs up beyond the preceding one.

The government just upgraded a traffic-light-controlled roundabout where I live, and enlarging the circumference in order to provide more queueing space was one of the considerations mentioned in the public consultation doco.

Also bear in mind how large, possibly articulated, vehicles have to navigate roundabouts versus how they can navigate box junctions, especially since the approaches to roundabouts by design slew approaching traffic to one side. (This is a consideration that for smaller vehicles brought about the idea of the mini-roundabout, and is also behind having overrun areas around reduced central islands.)