My personal opinion, 25's look atrocious on an SX. I completely agree with your thoughts to put 22x11x8's all the way around back on the bike.
First gear is intentionally very low for pulling heavy loads and creeping through rough terrain or climbing steep, technical hills. There is a possibility someone may have swapped in a Big Red rear end which has deeper gearing. The Big Red gearing is offset by the 25" tires. I don't know how much experience you have with these bikes so I don't know if your first gear is what it's supposed to be or not. The fact that it has 25's on it is a clue that it certainly might have a Big Red rear end. If it does then the next steps are up to you. As I said, I can't stand 25's on SX's, with the exception being dedicated mudders or lifted bikes. Even those aren't my cup of tea but in that instance at least the big tires serve a purpose.
I wish I could tell you how to find out which gearing you have but I don't know how. I'm sure someone will post soon with a suggestion of how to find out for certain what you've got there.
85 Tri-Zinger 60
85 ATC250SX
86 ATC250SX
87 ATC250SX
02 XR650L conversion
84 ATC 480R