Alternative to image sprite sheet?

We are planning on having thousands of sprite sheets and we need our app size to be minimal. Im wondering what options/alternatives there are for minimal app size with thousands of sprite sheets? For example, svg format has a small file size. Or maybe defer download of content to a later time (unfortunately that would require constant internet connect). Any ideas?

So far I have investigated ragetools for svg. But I can not get the quality animation we require with the rigging tools, and I'm having trouble converting swf to svg sprite sheet.

Thanks!