You need to have a data plan on your mobile devices to start or join a video meeting. When your mobile device is running in 3G or 4G/LTE mode, zoom app will only send a maximum of 1MB/min of data in accordance with Apple's guideline for bandwidth usage for a data plan.
For iPad, the active speaker will be in a non full-screen mode when transmitting in 3G or 4G/LTE.
For iPhone, the active speaker will be in full screen mode when transmitting in 3G or 4G/LTE.
[Update] In Version 2.0
For both iPad and iPhone, you can see full screen with sufficient mobile bandwidth. You can also see all attendees in slider view (iPad) or gallery view (iPhone).
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.