Tyrannotitan, with its small arms and a tall tail (ventral to dorsal), would be an interesting theropod to view in the wild. Much of its body would have been geared toward the posture of tyrannosaurs, on account of those little arms, but it would have probably had a posture much like other carcharodontosaurids, because it was related quite closely to them. The body postures of the two groups would be slightly different because the two groups were built differently, so how would an animal built much like one of the groups but possessing a similar characteristic of the second group, the small arm of tyrannosaurs, carry its body overall? All said and done, most images of Carcharodontosaurus show an animal that can rush forward and grasp prey, either large or close to the ground, with its head and neck extended to peer forward, giving an overall composition of an animal that holds its back nearly parallel to the ground. Tyrannosaurs, more often than not, are portrayed slightly off the parallel to the ground spectrum due to the fact that their massive heads created a vastly different center of balance; arm reduction or not. Perhaps the massively deep tail balanced the large head and small arm more, shifting the center of balance in Tyrannotitan back more toward that of its family group and allowing it to assume a posture more like Carcharodontosaurus than Tyrannosaurus.
The other consideration with small arms, as it has long been in tyrannosaurs with small arms, is feeding. How does an animal with minimalistic arms manage to feed itself? Regardless of the dinosaur-bird connection I find it odd that popularly we do not see more parallelism drawn between the feeding habits of hawks and other raptors with dinosaurs that have reduced forelimbs. My imagination, and it is quite vivid most times, can easily picture a large dinosaur feeding like a hawk (not for the faint of heart) with the exception that the teeth of a large dinosaur are replacing the tearing actions of the beak of the hawk. Much like in this image, a large dinosaur without the capacity to tear meat with is reduced forelimbs, such as those found on Tyrannotitan, could have stood on its prey as it ripped what it wanted away from the other meat on the body. Surely some use could be seen by the arms, but certainly not much in the vein of aiding the feeding process.
The other characteristic that really, as far as we know because we do not have the complete skull to help us determine how much of that structure is unique, sets Tyrannotitan apart from other animals of its time period is the extent of the ventral and dorsal chevrons and ribs of the caudal vertebrae. Though not many of these have been recovered those that have been show that the tail was rather broad top to bottom and, in the areas we know of, this would appear, if not paddle-like, then alligator like in its musculature and overall breadth. This indicates, and it may not be the only reason for the tail developing as it has, that Tyrannotitan may have been an adept swimmer. James Field has a rather nice swimming Tyrannotitan illustration. Despite the dangers of the water, perhaps swimming may have helped ambush prey or to simply reach prey living, or perhaps only nesting, in "safer" areas isolated from other types of predators during the vulnerable nesting or mating times of the year. That would be an awfully specific niche to control, but something would certainly prey on dinosaurs at those times and, if secluded by bodies of water was one trait of prey items during those seasons of the year, it would make sense that at least one predator would happen to adapt to gaining access to the secluded nesting and mating grounds. It s a theory I like to imagine has a bit of merit to it, though we may never know if that is true, of course.