You can shave off 2 more bytes by using em instead of rem. In your use case they are functionally the same. Rem is em relative to the root element. Your <main> pretty much is the root element.
But I think the original is still well into the land of diminishing returns.
I brought up rem vs em because the difference is subtle and if you're byte shaving it's "free" bytes.
As examples, I size page elements in rem, but text elements (usally), in em.
My main, content, article, header, footer, and aside should all generally reference page, and are in rem.
Main body text width, headings (h1 ... h6), super- and sub-scripts, code and pre blocks (often jarringly sized by default) reference the containing unit and should be in em units. Also, generally, figure and table captions, table text, etc.
I prefer to give figure or sub-element (tables, callouts, note boxes, etc.) margins and paddings in em as well.