She does look small compared to the jacket, but 1870s style dresses featured loose sleeves like these. The jacket also looks large because it has to fit over the bustle.
Girls of marriageable age wore her hair up. Were she much younger her hair should be loose around her shoulders, although as her hair looks a bit unruly she may have done something with it.
One more odd feature is that the photographer has messed with her eyes. Sometimes when a sitter's eyes lacked definition in a photo, the photographer would "enhance" them with little black dots. Usually with gruesome results, especially if the dots are uneven. Like here

So the eyes may be confusing her apparent age.
Cheers,
China