If you're watching them in Firefox, you can zoom in. Use Ctrl++ (which, strictly speaking, is Ctrl+Shift+= on most keyboards) to zoom in (i.e., enlarge), Ctrl+- to zoom out (i.e., shrink), and Ctrl+0 (zero) to reset. Or you can use the "View" → "Zoom" menu to achieve the same results. You can do the same thing in Internet Explorer (it lets you zoom to a user-specified magnification factor, unlike Firefox, which, AFAIK, only lets you navigate the hard-coded magnification levels).
I don't know specifically about WEBMs, but I do this with animated GIFs with no problem.
Obviously, there's a limit (300%? 400%?) to how far you can magnify an image before pixelation sets in. If the original image is too small, you'll hit that limit before you enlarge the image enough to make it properly visible.