She was just enjoying herself.

"Well, I was just enjoying the moment," Jill tells you. "I can tell this is what you want, though!" Jill punctuates her exclamation with penetration. You were ready, so she goes in easily. You thought she would feel thicker. She slides completely in on the first stroke. You sigh contentedly as she grinds her crotch into your butt. "Wow, you were ready for me! I didn't think I'd bottom out so easily in your bottom."

"I was expecting something bigger," you tell her.

"Be careful what you wish for! You might find that I am outside of your comfort zone half of the time!"