I usually use a real Android tablet, let me dig out my Chromebook ... hmm, does seem twice as wide as I'd like, even on my 11" screen (though that screen is big enough to render pages at a reasonable size even then)..

They seem to have thought all Chromebooks were comparable to laptops. I know there are some which are much smaller, and yes that sidebar would be too big on a smaller screen, but they don't have any "layout" options where they should be in Settings, so you can't change it. I'd suggest filing a bug report, or tell me your system specs and I'll do so for you.