Does a modern phone like an iphone 6 use more data than an iphone 4 to watch a video on Youtube?

Say I was to watch a video on Youtube, would a bigger and better phone use more of my monthly data to watch it?

Of course not. The data you use is the actual size of the video, nothing more, nothing less.

I don't think so. Should be the same. The bigger display doesn't make the video being watched any bigger. It just appears bigger because of that bigger display.