It's not even a corporate vs. small shop thing. Look at the team sizes on "giant" programs from big companies in the 90s and consider how enormous an undertaking they'd be considered by most software orgs today.
Like, I get all the Mythical Man-Month, Brooks' Law stuff and that that observation predates the period I'm talking about, but there's definitely been some kind of shift that has other factors causing this (with Brooks' Law compounding the problem). Not sure what it is, I just know that it sure looks like we're far less efficient at building software than we used to be (where "we" is the industry overall). It's like every single organization larger than a dozen people got way worse at building software, pretty suddenly, some time between about 2005 and 2010.
So as far as Google is concerned, the implied manpower requirements are a feature, not a bug.
Chrome's surface area is now huge (and still expanding at a fair clip), but matching Chrome's featureset has become table-stakes for being considered a viable desktop browser, and independently implementing (and maintaining) that entire set of features requires a correspondingly large developer team, even if you were somehow more labor-efficient.
Servo currently describes itself as a "web rendering engine", so I don't think they are aiming to become a full-featured browser, and I'm not sure if there is an important distinction between "browser engine" vs "web rendering engine". It makes it sound like they only want to focus on the rendering part itself.
Also it is a web rendering engine, not a full fleshed browser.