I have no idea if it’s the lightest but I bought the Zpacks hammock tarp used and it’s been great. Ounce for ounce, it’s hard to overstate how important good tarp coverage is for thermoregulation during a windstorm.
Not the question you’ve asked in the OP but I’ve slept under poncho sized tarps but never again. I would’ve been fine in a rain storm but having that wind cut through your hammock and quilts is not enjoyable.
Others here witnessed it but I slept under the Zpacks tarp w/ doors during a group trip to Surprise Lake a couple years ago and we were blasted by the wind pretty good. We’re talking about tarps flapping loose, stakes getting ripped out of the ground sort of winds. It was gusting hard enough that I was really worried that it was going to shred my tarp but the .51 ounce fabric held up fine.
Look at the dimensions when you are making the comparisons but the Zpacks tarp is the smallest I’ll personally go with moving forward. There’s obviously some point where you hit diminishing returns but for protection from the wind and how that contributes to your warmth overall, there’s a point somewhere that ounce for ounce it’s as important as down.