Responsive CSS design - large or small screen to start

Responsive CSS design - large or small screen to start

I plan to design a web app with Phonegap. As I need to deal with different
screen sizes, I will have to work with responsive design and CSS3 media
queries.
I have a 1080x720 resolution android phone, and I want to start with this
(large) resolution, and say the css is base.css. And for smaller screens,
I will add something like small.css to overwrite base.css (places where
needed).
Assume the css would be complicated, will it incur more workload for small
screen phones (whose capability tends to be weaker than large ones)? Is it
better to start with small.css as base.css, and overwrite it with big.css
on large phones? Or it doesn't matter in web app scenario (as css file
size is not a big issue)?
Any design considerations and hints?