Sometime ago I updated our cover art on YouTube, and I didn’t like how it cropped the image being too busy at the time I left it for another day. Well today was that day after a quick Google search I found the answer I was looking for.
The artwork image is displayed differently on each device that supports YouTube i.e. Desktop, Tablet, Mobile and TV. Read the full YouTube Channel Cover Art Guidelines here
Here is how your channel art is displayed on each device:
Channel View on TV
Channel View on Desktop
Channel View on Tablet
Channel View on Mobile