Eh, to be fair, if the 128 GB iPhone has 2 64 GB chips on it instead of one 128 GB chip, it would weigh more than a 64 GB phone, but not anything significant. The chips barely weigh anything and the difference probably wouldn't register on your average $20 postal or kitchen scale.
Yes, if the phone has higher storage it will weigh more but the weight difference is extremely small. Two feathers of the same size weigh more than 1 feather of an equal size, but you couldn't possibly notice the difference in your hand. Adding an additional flash storage chip would add some weight but just a tiny, tiny bit more.