Yes. It works as you said.
But why do you think that there is something wrong?
When request size if overflowed server skips request and post back specified message to client.
When you push STOP client sends AJAX request to server to cancel request.
In you case you push STOP after file upload started. And request wil not be cancelled on server side because it was already skipped. And 'File Size restricted' message has already be sent to client.
I think you mean that you have delay before 'File Size restricted' message.
It caused by browser. It hangs for some time if file is rather big.