If you add gzip compression, it's still smaller than doing the same thing with jQuery. "dart:html" provides an idiomatic DOM API where all list-like things are actually Lists, events are streams, Futures instead of callbacks, and so forth. It's pretty nice to use, but it's also one of the bulkier libraries.
Also note that you could now add a thousand lines of code and the file size wouldn't increase much. You already paid that one-time cost.
You can further improve the size by using a better minifier on top of --minify.
Ah, that makes sense re:same size as if you added jquery, and the API abstractions do sound better.
Though as a counterpoint (and relevant to angular 2 :) I read they're going to just use the raw dom api's instead of a jquery/jquery-lite abstraction layer as the dom api's don't need the smoothing over in modern browsers like they used to.
Also note that you could now add a thousand lines of code and the file size wouldn't increase much. You already paid that one-time cost.
You can further improve the size by using a better minifier on top of --minify.