Spirit runs with a 28 inch seat pitch, which is often physically shorter than the distance between someone's rear end and knee caps[1]. It's an unusual choice. Most other low cost carriers have 31 inches or more. I'm not sure Dollar General is the right comparison.

[1] Once you account for the padding, the angle you have to sit at, etc. Even if you aren't unusually tall, it is noticeably awkward and cramped.