Currently syncing stage view to participants joining the live stream of the Webinar (Webcast) is not support.

https://help.webex.com/en-us/article/nic6kmbb/Sync-your-stage-with-everyone-in-meetings,-webinars,-and-events

The caveat in the above article states the following: Your synced stage doesn't appear for participants who join using a video device or from a live stream.

Since the Webcast is a live stream, it would not be synced and take the default scale.

Additionally, moving to stage view is not supported in Webcast as listed in this article: https://help.webex.com/en-us/article/4tyard/Move-participants-to-the-stage-in-meetings,-webinars,-and-events

"This feature is not available for webinars in webcast view."

Customer wants to be able to sync the stage view with participants join the live stream (webcast) as well.