> The reason the others you checked are of the same size is because they are forks of the same two browsers.
Exactly. What's the alternative? Safari is nice if you're on macOS/iOS but isn't available elsewhere. Every other browser that has any semblance of market share is a fork of Chromium and shares its size. So I'm not sure how Firefox is "overstuffed" but other browsers are not.
> The complexity of an operating system, even a simplified one, is of a completely different sort and depth.
I agree with this, and it's obviously an exaggeration, but I don't think it's an invalid comparison. The API surface area that browsers need to support is enormous, and they have even stricter backwards compatibility guarantees than most modern OSes. Any browser worth using is going to be pretty large simply because they're complex beasts.