Polymer offers a lot of UI widgets, which are enormous when you glob them together, but the base polymer library is actually incredibly small due to the fact that's it's leveraging the browser's native custom element functionality.

Can't reply to your comment so editing here:

Works natively on IE11+, Edge, Safari 9+, Chrome, Opera, Firefox, and mobile browsers. The polyfill for everyone else (webcomponents-lite.js) is 41kb.

Ok, it has been some time since I checked. Don't you need lots of big polyfills ?

No - 95% of time you need that webcomponents-lite to get things working.

Thats completly untrue - polymer is close in size to jquery and a lot smaller than react core for example.

No, it's a little library. And smaller in Polymer 2.0. :)