This was the default behavior a while back, but since then, it has been removed, due to irregularities in the sizes of descriptions, and titles throwing off the layout of the gallery preview.
Perhaps this can be added as an option, but you'll have to be aware of when a user names an image something like "AAAAAAAHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!!!!!!!!!", that the preview will stretch to fit it, and it won't be pretty.