[Problems broadcasting on iphone]

One solution is Download youtube videos to iphone, read https://www.macworld.co.uk/how-to/iphone/download-youtube-videos-iphone-ipad-3636901/,  to verify if your problem is the internet connection, your mobile data may have run out.

The other is to verify if the youtube app you have installed is the most updated, if not, update your app

[Reduce the size of a file]

To reduce the size of a file you can use an online convert to reduce the size of your files. You can read download online videos in the following information: https://www.vidpaw.com/blog/savefrom-net.html

[Problems with video playback]

Sometimes that happens with youtube or other streaming sites, that is why I recommend you read how to download youtube videos here: https://en.savefrom.net/1-how-to-download-youtube-video/
With this you can have your favorite videos on your computer without the need for your internet bandwidth.