just beceause it won't grow to it's full size it's still a fish that needs to be cared for,but if the fish was only 4" when you moved it from the 29 i can't see that causing it,so it must of been water quality
So it went from 4.5" to 5" in one year in a 75 gal. I agree, it's probably water quality. How is your filtration, water chemistry, and how often do you do water changes?
2 years old and 5" its not so bad not all rd are going to be 12" so u might just have gottin the runt dude it might max out at 8" its just a petiet lady thats all.