Comparisons with the past are meaningless because browsers are far more complex (and performant, and secure, and versatile, etc) than they used to be.
And sorry, but your gut estimates are not likely to be that accurate if you don't work in this area.
Browsers have evolved - because standards did, and expections along with them, but in the past they too were quite complex(see also Netscape's play: browsers, email, usenet client etc, suit) and again, it was harder then that it is nowadays to build such systems.
Even an HTML5 parser is harder than it looks to implement to the level needed for a browser. You have to build a proper DOM, you have to be secure against all kinds of fuzzing, you have to implement off-main-thread parsing and speculative readahead for performance, you have integrate it correctly the HTML5 event loop for document.write() etc, you have to support innerHTML and so on.
Likewise building a JS engine is impressive, but I do not believe a Web-compatible competitive-performance JS engine can be built by any single person.
Nothing about that or the JS engine is impressive really. That was my point, more or less. Of course, there's a difference between building something that works, and something that works exceptionally works etc etc(all the stuff on top of that), but all told, I still don't think building a browser justifies assembling such a huge org, even if there is no reliance on third-party technologies.
Implementing a fast HTML parser that will produce a fast DOM is a little harder.
Tying it in with a fast CSS engine is harder still.
Tying it in with a JavaScript engine is a bit more work as well.
Implementing layout is moderately fiendishly difficult to do and get right.
And it just gets harder the more of the web platform you add, and the more it has to be fast.
The big difference with most existing browser developers is the majority know a lot of the web platform inside out—but that's mostly a matter of having worked on an implementation for a long time and having to worry about edge-cases and interactions no web developer would ever think of. There's certainly a few who have achieved a huge amount for individual developers—but that's not a trait unique to the browser sphere.
Scope differences make it incredibly hard.
Plenty of people on the Blink team work on things like Skia, which neither the WebKit nor EdgeHTML teams have equivalents of (because they just leverage OS-specific APIs), or on their HTTP/TLS stack (again, neither WebKit nor EdgeHTML teams have equivalents of), etc.
Gecko is somewhere in a middle-ground; they rely on third party libraries (including Skia—does that make that part of the Chrome team Blink/Gecko developers?) but not quite as much as WebKit and EdgeHTML do.
Core web stuff? The Blink team is larger, yes, but by how much is a hard question.
FWIW, I believe the Gecko team is nowadays larger than the EdgeHTML team, but they're certainly close in size. (And close to the size of the old Presto team, which admittedly had a scope closer to that of the Blink team.)
I imagine to some limited degree it's true for WebKit (though I suspect they don't have anyone whose sole job is infra) given Apple don't have any general public bugtracker or CI system, and totally untrue for Edge.