Why would I want one that small?

Recently me and my gf got into ass play/pegging. We started off small but we finally decided to buy a strap on. Obviously with limited experience I didn't want a huge one so my girlfriend looked around for a few on Amazon or specific websites that sold sex toys. Eventually she calls me over and the conversation went like this.

GF: "Babe do you like this one?"

Me: "I know I said I didn't want a huge one but why would I want one this small?"

GF: "*Heavy laughter*"

Me: "What's so funny?"

GF: "Look at this." She says still laughing while pointing at the text on the screen. The text said 4 inches which is slightly longer than me. Seeing my reaction my girlfriend laughed even harder before we decided to buy a slightly longer/thicker one.