I think you're designing something that's too large to fit into the web browser preview, so it's being scaled down. That's causing the quality to look worse.
Your top screen shots is almost 1700px which is well beyond the normal desktop size we should be designing for. While some people have screens that large, most people have screens 1280 - 1440 and that is what we should be designing for. I really dislike how XD's default desktop size is 1920px which is something I don't recommend using as most people do not have a screen that large.
Adobe Certified Expert & Instructor at Noble Desktop | Web Developer, Designer, InDesign Scriptor
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