Upgrading to rollup 3 could slightly decrease the bundle size for CJS with --treeshake:

Rollup Repl 2.79.1

Rollup Repl 3.2.3
removed Object.defineProperty(exports, '__esModule', { value: true });

There're a few built-in rollup plugins in tsup currently prevents our upgrading:

• @rollup/plugin-json: 👍 5.0.0 available
• rollup-plugin-dts : 👍 5.0.0 available
• rollup-plugin-hashbang: ❓ need investigate