Especially because it's mostly relatively thin until you get close to the base--you could sit on that and still not have the thickest part actually in. If I had to guess, that'd probably be around.... 6-7 inch circumference at the base? Big, but because it's the bottom of a taper it's not nearly as impactful as like the same circumference on the bulb of a butt plug or a knot.
Now, whenever I admonish my kid to ask before borrowing other people's toys, it will forever be with a confusingly suppressed laugh. He's going to get the wrong message from that for sure.