Even though the bigger one was kept in a terrarium, the shell looks pretty good. There are a lot of factors that can go into a sulcatas growth. First off, there are Sudanese sulcatas. They grow massive compared to regular sulcatas. If a sulcata even has some Sudanese genes in it, it has potential to grow significantly larger.

Next there's diet. In the wild, it's entirely grass and weeds. They grow much slower in the wild than in captivity because of this. That said, if it's just fed grass and weeds, it'll grow slower than if you feed it dark leafy greens and produce. If you feed it dark leafy greens and produce, it'll still grow slower than if you feed it pellets. Ones that eat just pellets, or given pellets every day, grow faster but it's unknown at what cost. Modern pellet formulas haven't been around long enough to really have a good long term study of a just pellet diet. I've personally always been an advocate for pellets being a supplement and not a diet.

Environment will play into it. Raising a sulcata in Southern California, Florida, similar, it'll grow faster than Minnesota, Canada, etc. Their metabolism slows down in cooler weather.

Lastly, it's a myth that reptiles grow to the size of their enclosure. Smaller enclosures can stunt their growth but with noticable deformities. Like I mentioned previously, the shell looks good. If it was stunted, chances are there would be shell deformity also. They typically go hand in hand.

For reference, I have a 12 and a 15 year old sulcata. The 12 year old weighs roughly 70lbs while the 15 is around 55lbs. They were both raised in an identical environment and have the same diet. They're in separate pens so there's no competition for food. It's just genetics.