**putting on my Civil War historian/professional horseshoer's hat**
Just for the record both humans and horses today are, on the average, much larger than our 19th century predecessors. Horses more so because while our increased size came pretty much by accident from better food, environment, and health care, our equine friend's new stature was no accident coming from 140 years of careful breading.
The "adjustment" has not been equal over time. While we are probably 10% larger than 19th century people, some breeds of horses are as much as 30-40% larger.
Look up some pictures online and see. Or if there's a good museum nearby visit it. You'll be surprised how small the uniforms and saddles are. In fact for many, many years lot's of Civil War uniforms in museum were miss-identified as "musician's" or "cadet uniforms" because they looked like children’s clothes. In the last ten years a lot of those have been corrected.