Why did Emillie prefer to ride Joey than topthorn? How did topthorn fell?

Joey was not as wide and tall as Topthorne. Emille felt safer on Joey.

Topthorn just taller and broader than Joey so she would find it difficult to mout him and even more to get down.Sometimes she would use me as a stepping stone to topthorn, hut it was a difficult maneuver for her, and more than once she fell in the attempt