Protoype 1440 on macrpo - not scaling to screen width

I have a 1440 design when viewed in chrome browser on
Macbook screen is 1496 x 967

If viewed at 100% there are gaps at the side - which I presume us (1496 - 1440) = 96px
I wish to view full screen so my design fills the edges, however choosing other options e.g. fit to screen = makes design very small
or fit width = looks same as actual size

My design frame has clipping selected. The browser view is at 100%
I have also tried in Chrome and Safari with the same results.

Any ideas how to solve this please?