And there's the rub- having to modify one to achieve the same weight as the other in stock form. Not a fair comparison.
Throw the same parts and money at both, and you will still be back to the original weight difference (or very close to it).
An aluminum-headed small block will still weigh less than a similarly equipped big block, providing even better weight distribution and inherently better handling.
BTW, I'm not dissing big blocks, I've had many. Just saying that if you use the same type parts on both engines, the SB will always be lighter.