Youtube channel manager expert.

Hello, I am in a process of launching a large youtube channel which will be uploading videos shot using mobile phones. I wish to hire someone who can help me in managing multiple youtube video related stuff and the project has sound budget. Some of tasks are but not limited to replacing chroma backgrounds, color grading natural videos, video SEO and more. You dont need to be knowing everything as I am aware that this is not going to be a one mans job. But before I move ahead with my project I am facing a very basic issue which might be a simple solution for a person who is managing a youtube channel which hosts videos shot using mobile phones. If someone here on fiverr can help me with a suggestion on this it will be great as it will allow me to further document my project scope and come back and share it here so that we can take it further. In order to reach a conclusion I need to resolve this issue which will help me take further decisions. This is what is happening with me: There is a term known as window boxing for youtube videos. It means black bars on all four sides of the videos. As a test I have successfully uploaded 1080p and 720p videos on my channel multiple times. I see that when using a mobile to watch the videos on a high speed internet connection, youtube displays it in full screen but when the same videos are watched on a different and slow speed connection it auto displays it in a window boxing display. What is frustrating is that I see many channels which have used 1080 or 720 just like what I have done and their videos seem to show up in full screen even on a slow speed connection. No window boxing in their cases? Why? Am I missing out on something? I have maintained all the guidelines of youtube settings for aspect ratio and resolution. I have googled for this issue for almost 100 hours in total but did not find a post which throws light on a similar situation. Your involvement is much appreciated. Thanks,