In the demo blog post I linked, it looks like libraw is around 3MB, but that the blogger who wrote that post didn't pay much attention to eliminating cruft and the script is additionally not minified. Minified and compressed it's under 450k.
http://dev.tag.is/rawson.js/ seems to be an emscripten-to-JS compiled dcraw.c, and while less functional than libraw it's under 500kb.
http://dev.tag.is/rawson.js/ seems to be an emscripten-to-JS compiled dcraw.c, and while less functional than libraw it's under 500kb.