Given the DRAM refresh is part of S3 standby, I'm afraid this is circular reasoning.

I suspect this is one of those "it depends" situations; does the 128gb vs 64gb sku have more chips or denser chips? If "more chips" probably it'll draw a tiny bit more power than the smaller version. If the "denser" chips, it may be "more power draw" but such a tiny difference that it's immaterial.

Similarly, having more cache may mean less SSD activity, which may mean less energy draw overall.

If I had a chip to put on the roulette table of this "what if" I'd put it on the "it won't make a difference in the real world in any meaningful way" square.