Where are you uploading it to and what program are you using to upload it. Some platforms such as Wordpress may compress the file even further. One way to tell if your host or design platform is compressing it is to upload it and then right click on it and download stand compare file sizes.
Screenshots showing the difference would also help.
Can you explain what you mean by loose quality (this can mean different things to different people e.g. pixelated, colour change, blurred etc) . Useful would be a screenshot from the save for web preview (at 100% zoom) and a screenshot from the uploaded website.