Given some airlines (Hawaiian?) do sometimes weigh passengers on their smaller craft, I could believe fuel load would drop if pax weight dropped but I don't actually believe either fuel cost or weight really strongly informs pricing. It's used as an excuse. A350 are significantly more efficient than older craft, but if you do more business suites you carry less and make even more money. The price does not drop, profit rises.

(The weight is about balance and total load as I understand it)