Created attachment 5599 Comparison between icon in window buttons and task switcher. The default icon or general window icon which certain applications, such as urxvt and mplayer, uses does not seem to scale up correctly in the window buttons plugin for the panel. The icon scales perfectly well for the task switcher (alt+tab) though (see screenshot for comparison). I've tried resizing the panel, but the icon seems fuzzy at all sizes. Running Xfce 4.10.1 (xfce4-panel 4.10.1-1) on Arch linux.
This is a downside of relying on libwnck - it only provides icons in two sizes: 16px and 32px. So oftentimes it will be fuzzy. Like in your case, where a 16px icon is upscaled. Nothing that can really be fixed in the panel - especially because there is no good alternative to libwnck.