McCaig talks about designing Padme; about writing a story by "flipping it" like artwork, and gives the example of making a woman the main character because it's done less often; and he talks about trying to make narrative drawings based on Lucas' really sketchy initial suggestions and guesswork.
I put all this together to get; with no actual script to work with, McCaig makes some educated guesses, and "flips it," so Padme is his protagonist - which is why he focuses on designing her.