I don't really know will such things exist. I own a Ashdown LB30, is a 30watt all valve head. I won't say is very light and is not too small either. Of course comparing to bigger valve amp the LB30 is tiny and light weight.
I can see the size could be smaller but I don't think the weight would be light. Never say never on technology, it may have something in the future that will full fill our requirement.