You're absolutely right! Sketch and Gulp don't add to the final size. And HTML and CSS are needed.
So let's be generous! I'll assume that 4x the core content size (1.3k) is needed for markup and presentation, for a total of 6.5k. Round a bit... that's still 5100% inefficient.
I can see why some might consider that a bit inefficient. Of course, I can also see why some might consider the design the most important part of the site.
Sketch doesn't add to final size. Neither does Gulp. Sass and Nunjucks might, but it's a website. They tend to need HTML and CSS.