I really don't know how to show you that they grow that fast. However, I have some photos of mine. I have had him since September of 08. He is twenty six inches long, maybe a little more, now. What people don't realize is just how big they get -- not only in length, but also in width or girth. These are big, bulky fish. They eat a whole lot, which means that they poop a whole lot, which means that the filtration needed for any tank they're in is pretty serious -- 10x volume is usually standard.
I know you're asking for proof, and I don't have any, really, but I wouldn't lie to you. I think my RTC is wonderful, and I wish they were easier to keep, so that people could enjoy their RTCs as much as I do. Unfortunately, it's just not easy to house and feed these guys. That's not taking into account the water bills (water changes of 250 gallons or so weekly), electric bills (big pumps cost money), and all of the other necessities that come along with having a huge fish.