The HTML template (index.html) is different for preview and publish. Try saving preview index.html and then replace it in the published folder.
Whoops! Looked in preview html - it could be unsuitable for upload - there's some *.js scripts attached.
You should tweak some CSS to get border and make same center vertical align as in preview html file.
As for fullscreen - your game runs still in window, but just without fancy borders. I'm sure that exported page is more suitable for different pc configurations, though could be a bit fancier - like one on the preview.