I haven't been part of a small team building either, but the obvious small team doing RDBMSes is SQLite. Three people building the world's most popular software, and possibly the world's most robust database. They found a niche and implemented it perfectly.

For browsers, the most prominent example is probably LibWeb/Ladybird, part of SerenityOS (but with a Qt port, allowing it to run on other operating systems). I've never tried it myself, but supposedly it's complete enough to run large web applications fairly acceptably, which is amazing for something developed largely by basically two people AIUI.