preview pixelated

hi, im having a problem with my preview

i want it to be full hd, but when i press play, it gets pixelated? ive attached both a paused screenshot and a screenshot while playing the preview, and as you can see, it changes quality 

i have my resolution set to full, so idk why this is happening? i even set the 'adaptive resolution limit' to 1/2 (instead of 1/8) in my preview preferences