Just because 5’8″ ~145lb is small to our standards, does not mean they would be anything but superhuman physically compared to the current average american.
The civil war soldier lived a life 1000x more physical than one in the modern era.
I’ve seen plenty of laborers (and wrestlers) around that size that’d blow your mind, not only having endurance, but considerable strength.
The average civil war soldier lived rough. A good deal of miles marching daily was par for the course. Hell, life at the time involved way more walking.
Before soldiering it was manual labor whether they came from the city or a rural area. And manual labor makes you strong.
We think him small, but he wasn’t weak.