My phone has 256gb, plenty for everything on my phone, yet when I try and watch videos I recorded with my camera about a year and 3 months ago and further back I have to press 'Play cloud video'. Even then it takes forever and usually has to buffer half way through. Why is it not watchable instantly on my phone like when I recorded it?
Check if you have synchronization enabled with OneDrive. This happens if you have enabled synchronization with OneDrive and at some point clicked on Free up phone space.