When using my iPad to stream tv programmes why can’t my Apple TV allow me to watch a full size screen on my th

content providers has the option of blocking airplay if they so choose

netflix does this

https://www.theverge.com/2019/4/6/18298146/netflix-confirms-it-yanked-airplay-support-wont-let-you-beam-shows-to-apple-tvs-anymore

iPad Screen Mirroring projects a 4:3 source to a 16:9 display, which will require pillar bars.

Apps that have in-app AirPlay functionality will be able to send just the source video, which is probably 16:9.