Capture One used to suffer from exactly the same issue at some point in V4. Very unrealistic sharpening at anything other than the 100% view. The issue was addressed in a point update to correct the over sharpened look. 100% view is the correct way to judge sharpness but unlike what Yair says a preview should be a preview and not a complete miss representation of the actual image.
I would log a support case about it and use Capture One if it becomes a real problem.
I think "complete miss representation of the actual image" is a bit harsh...Not sure how much experience you have with Leaf Capture but this is the first time I here of someone querying about it...
The way the preview is drawn on screen also depends on the screen resolution and how much of it the preview area actually takes. You'll notice that if you change the size of the preview area by dragging the divider, the preview will be re-adjusted and sometimes dragging it a few pixels to the left will sort things out. (this may work a little bit different between landscape and portrait images).
One thing that LC does well is generating the previews very fast (faster than any other raw converter I believe) which was done following many users' feedback. I guess that there's some trade-off here for specific magnification settings
Yair
Yair