If it's 8 inches at 3 years old, I don't think it's stunted. It could just be a female--8 inches would be a pretty healthy size for a female Midas, I think.
What you'd look for is--I'd say "proportions". Is the fish thick, and stocky, or lanky and kinda skinny? At 3 years old, a Midas should look like a tank, a full back.
Is the fish's eyes in the right proportion to the rest of its body? If its eyes just look a little too big, or buggy, and that's because its body just hasn't really filled in completely yet, it's "stunted".
I'd say after 3 years, you pretty much have what you have with this fish. If it looks good and thick now, it may get a little thicker. If it looks kinda skinny and under developed now, then it's been dealing with very adverse conditions for years. that takes its toll.
The fish will certainly do better in a 120 after living in a 55, and it'll show after 6-12 months. But don't expect that it'll go in your 120 Gallon, and put on another 6 inches and become a MONSTER because you put it in a bigger tank.
Perhaps it'll put on more girth, and grow another inch or two over the next year, in a bigger tank? I'm guessing, but that seems possible.