"Proportionate response," as it has come to be used lately, means that a nation with more potent military forces than those of than its enemies should give the enemies a "handicap," just as a better golfer gives a lesser golfer a handicap in a country-club match.
Israel, to name the nation in question, has a potent military force precisely because it is surrounded by enemies who would destroy it. Israel is most decidely not playing a country-club golf match or playing a game of tit for tat. Israel is fighting for its survival. It needs every ounce of military might it possesses, and then some. Proportionality, in this case, should mean the application by Israel of enough of its military might to ensure its survival. Israel cannot afford to err on the side of caution.
What about the civilian casualties that Israel inflicts -- incidentally and unwillingly -- as it defends itself? Regrettable, yes. But what about the civilian casualties among Israelis if Israel fails to defend itself adequately? Or don't the lives of Israelis matter? Apparently not, to those who cynically cry "disproportionate response" as Israel defends itself.