1. Small, fast and minimal.
2. Easy to create plugins.
3. Budding ecosystem and explosion of plugins.
4. Conflicts ensued and some plugins got pulled into the core project.
Eventually, the growth of jQuery tapered off as the project stabilized. Not only did the size taper off, it got smaller as well. After nearly 10 years, we're talking about a payload of 30K minified and gzipped.
https://mathiasbynens.be/demo/jquery-sizeThe cycle of bloat doesn't always take hold if your team is disciplined and dedicated to keeping it small. You simply can't expect to your 3.0 to be as small as your 1.0 because it's very unlikely you're going to know upfront how people are going to use your software.