From my limited understanding of the situation, Japan was of the mindset that they were gonna fight to the bitter end and never back down. Dropping the A-bombs was like a wake up call that there was no point in going that route. So I definitely think there's legitimacy to the argument that dropping the bombs saved lots of lives on both sides. I still don't think that makes it proportional. Proportional means keeping your response on a similar scale to what the enemy attached you with, not choosing whatever path will hasten peace and limit death in the big picture.
That's why I said that the argument for dropping the bombs is the argument against proportionality. It depends on the situation, and proportionality is the appropriate response in some circumstances. In the Israel example, Hamas can shoot a thousand rockets and likely kill no one, so a disproportionate response causes more total death, not less. But if Hamas was actually killing hundreds/thousands of Israelis when they launched their attacks, Israel would be justified in initiating a steep escalation IF it could swiftly force an end to the violent conflict and prevent a prolonged, deadly exchange of attacks. I don't think anyone has an obligation to remain proportionate if that will only lead to both sides continuing to slaughter one another.