Exactly, your mass increases much faster than your frontal area.
Assuming "not all the extra weight is fat" that leaves a lot more muscle propelling not that much more frontal area.
A second benefit is that once up to speed you are a lot less affected by gusts since mass in motion tends to stay in motion.
Hills are another story.
I'm 5'10", 205lbs, and my shoulders measure close to 2ft across. Those headwinds suck. They suck even more with my laptop backpack.
Lean over the handlebars, downshift for a bit, inertia is your friend, and feel free to get pissed off at the wind. Combined, it works wonders.
(And people wonder how I've lost close to 30 pounds this summer...)