You need to have a data plan on your mobile devices to start or join a video meeting. 

For iPad, the active speaker will be in a non full-screen mode when transmitting in 3G, 4G/LTE, or 5G. 
For iPhone, the active speaker will be in full screen mode when transmitting in 3G, 4G/LTE, or 5G.

For both iPad and iPhone, you can see full screen with sufficient mobile bandwidth. You can also see multiple attendees in gallery view.

If the bandwidth is low, it will automatically scale to a smaller screen. 

Note: To conserve data usage on mobile plan, you can turn off video. Or jump to "home" screen, iOS will put the app into "recording" mode which turns off video automatically, preserving audio.