I don't know if your figure of 40MB includes support for javascript, but if Servo offers native DOM apis that would let you omit all js-related dependencies. (I don't know if it will, hence "if".) Even at 40MB, with static assets like html templates, css, and images, and a compiled binary, you could probably package a full-featured app up at ~60MB, less if compressed.
I wanted to know how that stacked up against some of my favorite and most used apps on windows, so I made a list: https://pastebin.com/KnkFcJXh (sorry, it's a bit long for HN).
I'll be honest, if I could publish a native app that's blazing fast, is cross platform, and whose size sits between Paint.NET (40) and VLC (130), and is even a fraction as useful as those two I would be downright ecstatic. I know for some people the final build size is a very important metric for native apps, and to those people I'm sorry. But if it's under 100MB and it uses about the same amount of ram I'm actually pretty ok with it, and I think a lot of others would agree with me.