I've seen some references to this behavior, but haven't seen much that addresses it directly, so thought I'd post my observations for consideration. Of course if anyone has a fix please let me know.

So far, I tried replacing the HorizontalSlider thumb image by editing the tundra.css file. My image is 8px wide, 14px tall (versus the default image of 16px by 16px).

I changed the CSS entry ".tundra .dijitHorizontalSliderImageHandle" with the new image size and url. I also changed the CSS entries ".tundra .dijitSliderFocused .dijitHorizontalSliderImageHandle", and ".dj_ie6 .tundra .dijitHorizontalSliderImageHandle" with the new image url.