So I just finished resizing the big T down to medium and large size, and I still think each of these are in the 15+ CR rating. Isn't the statted beast as is too low for all its special abilities?
Not sure what version you are using, however unless you did major revision to the monster its CR appears to be in the range of 55 to 70, due to hit dice and special abilities. The big T is harder to kill then an avatar of a deity.
49+ HD (Colossal)
"Small beings can have small wisdom," the dragon said. "And small wise beings are better than small fools. Listen: Wisdom is caring for afterwards." "Caring for afterwards ...? Ker repeated this without understanding. "After action, afterwards," the dragon said. "Choose the afterwards first, then the action. Fools choose action first." "Judgement" copyright 2003 by Elizabeth Moon
Just the 3.5 MM1 version. I look at all the special abilities and the epic weapon DR and think CR 20 is far too low.
It's lower than 20. A party will have the resources to kill it by the time they reach 17th level. Earlier, if they can get their hands on a Wish from something.
"The Wild Mages I have met exhibit a startling disregard for common sense, and are often meddling with powers far beyond their own control." ~Volo "Not unlike a certain travelogue author with whom I am unfortunately acquainted." ~Elminster