If you get in until your chin, her ass would probably be tight enough that it'd pull you in to your shoulders. Don't even have to push anymore, just let her muscles work you deeper into her body
The way he's pushing the top of his head against her ass makes it look like hes trying to shove his head up her ass, I feel like her hips are actually wide enough to fit an adult head inside of her, she'd just need to stretch out her hole a lot, I would love to see a video of her stretching her asshole out, then he inserts his head into her ass, but of course that isn't going to happen lol, but if I were that guy, I would stretch her ass out and slowly insert my entire head into her massive behind.