The Safari/WebKit team is much more reasonable, with probably an average of one to two hundred people working on it at any given time (this is a bit complicated to measure, since many people move around between the browser and related teams like Mail or CFNetwork–and not to mention the various non-developer roles that only work part-time on Safari–but it should be accurate to an order of magnitude).
I'm sure there are lots of other engineering teams at Apple focused on security, build systems, developer tools, infrastructure, etc that the Safari/Webkit team relies on.
Then there are all the non-engineering functions like HR, marketing, etc.
Mozilla has to do all of that.
I'm including all of that in that number, the actual organization's full-time engineer count for it is probably half that. Also, I am excluding people working on things used by Safari but general purpose and part of the OS, such as system libraries and platform security features (many of which Chrome and Firefox use themselves).