For comparison, ClojureScript, another language that compiles to JavaScript, adds to the bundle size, but in my experience, it's mostly because of the runtime (core functions and data types) because the Google Closure compiler optimizes the initial transpiled JS.

I would guess running the TypeScript output through the Google Closure compiler (or adding similar optimizations directly to the TypeScript compiler) could make TypeScript bundles comparable to JavaScript bundles or smaller. It looks like people see significant improvements running TypeScript output through Closure compared to just WebPack [0].

[0]: https://medium.com/appmonet/using-closure-compilers-advanced...