This is perhaps one of the advantages of being on the smaller side. I'm 5'9" and rather slender so I really don't get much value out of domestic first class. I remember my first Spirit flight and having heard about how terrible the seat pitch would be, but realizing it really wasn't all that bad at all. I could still stretch my legs out completely if I didn't put anything below the seat in front of me. I probably would have had a very different experience if I was over 6'.
I wouldn't say I don't care about upgrades; I enjoy them and appreciate the extra space, but I'm sure I don't appreciate them as much as someone taller would.