I've seriously considered having one done on Mamou, but have to hold off at this time.
I think that it's very cool that you had one on Sparky, and will be very interested in the results.
May I ask what company you went with?
Most cats do grow a bit after 6 months old, and most fill out at some point.
Uhura, for example, is not a large cat. But, she was about 5 before she began filling out to become more blocky than she was. She was always wiry and muscular, but now she's heftier, and still muscular, with no excess fat to be found.
Cats who have less consistent food, such as strays, will grow at any point that food increases in quantity and quality. I've gotten 1-4 year old strays who grow considerably after we get them. And, not just fill out, but grow in other ways, which seems odd but it's true.
Mamou was an example of that. He was not a tiny cat when we got him at a year or so old, but he became one of our largest cats ever, next to Sailor, and the Brock/Badger/Friar Tuck/AquaNinja clan, over time.
Mind you, you are limited by genetics. A well fed small breed will only get so large, but when few a regular diet of high quality food, and enough of it, they will meet the high end of their genetic potential, unless they have some health problem that I'm not taking into consideration.
Uhura got both dry and wet foods when she lived with Ds, and she ate 'wild food'. She was always shiny and healthy.
It was only last summer that we took over her feeding, as she wouldn't go near anyone but Ds. (Now, she sleeps nearly through the night against my legs, and will recline 1/2 on me and 1/2 next to me - under my right arm, for a while each day...enough so that she sometimes interferes with my typing!) But, we indulged her with extra wet food, since the 'boys' would chase her off back then, and we began giving her buffalo meat. Somehow the addition of the meat, and extra 'treat food' increased her body mass.
I don't know if she stayed muscular va. becoming overweight because of the type of foods that she gets, if it's because she still races up and down a hill, etc., or if it's a combination of the exercise plus diet, but she exercised the same way before she filled out, so I feel that diet has been a larger part of it.
In the end, I expect that Sparky will look larger at three years old. Maybe you could get a couple of photos of him now, then in a similar pose at a year old, and again at 2 or 3. We don't always see the changes when we see them on a daily basis, but photos really do tell the story!
One way that I've had success with that, is to choose something like a living room chair and get a photo of the pet on that. Then, get the photo of the pet on that chair each time.
Love All Life, Thank You For Posting! :)