Actually, SVG previews _are_ rendered/displayed at the size the SVG file specifies, but only if that size doesn't exceed the viewer's image size limit (specified in their preferences; I think the default is 800x600px). Larger images are previewed at a smaller size that won't exceed the limit instead.
I dislike to be piggy, but thus they are NOT. Only if smaller than 800x600! I did not know that this is a default wikimedia-limit (which a user can change). And it would be also possible (whether useful we can discuss) to remove this restriction and ALWAYS render them at the given SVG-size prominantely. (browser will show (the lower left) part, but you can move the mouse-pointer to see the whole image, or browser can scale it (the pixel image) down).
Thanks for pointing out the exact algorithm for showing users *this* size.
Achim