16x9 ratio: story size matters?

Hi,

I have found that a 16x9 ratio gives me the best results on multiple devices.

Therefore i always use a storysize adjusted to 1600 x 900 pixels. A colleague of mine suggested to downsize our template to 720 to 405 (also a 16x9 ratio).

In my opinion i bigger is better because fit for screens/settings using high resilutions plus i think that downsizing content is always a better option than upscaling smaller content.

Is this true? 

What could be reasons (apart from doc size) to work with a smaller version of the 16x9 ratio?

Thanks in advance!