I am not sure what the cause of this issue is, however my images appear to loose a great deal of pop, and luster when downsized to 850 max pixel width. Perhaps you can't scale down all that detail and keep it sharp with so much down sampling, I am not sure.
As to downsizing. There is a difference between a lack of "pop" and a shot not being sharp. Usually if a shot has "pop" it's bright and vibrant. Softness or sharpness is something different. All 3 of your shots seem to pop just fine. As for sharpening, it is normal to apply a light sharpening to your images after downsizing for the web. I have been doing it for years now like most others, and it doesn't matter how many MP a camera has, it's just a normal result of downsizing that a shot will lose it's sharpness.
