Assuming that this isn't just crappy merch-art that couldn't be assed to keep to scale, by far the most interesting thing about this is that Grimlock appears to be about 2 1/2 to 3 times larger than a real T-Rex, which Prime (going by the previous films) would be either at eye-level with or a bit taller when it's hunched-over like that. All the Autobots are supposed to be getting major design-overhauls for this one, which is supposed to be a break from the story and human characters from the first three (no one has seen a good image yet, but the chatter has been more solid-looking bots with fewer moving-bits and exposed parts and they've said Prime is blue now) but Prime was confirmed to still be a truck and even if not it's hard to believe they'd be shrinking him down to near human size. The simpler answer is likely that Michael Bay is exactly the filmmaker who would look at dinosaurs and say "No, no - we need to make them BIGGER!"
Against my better judgement, I'm holding out a certain amount of hope for this one. I really do regret the immature cheap shots I took at Bay (absolutely as a person and, to a lesser extent, as a filmmaker) back when Escape to The Movies was still finding it's voice; and "PAIN & GAIN" was a potent reminder that whatever you think of his aesthetic predilections he really is something when he wants to be. The previous three Transformers movies all failed at least in part because of the tug of war between the Amblin-wannabe Sam story he was stuck with and the stuff that actually seemed to interest him - maybe (maybe!) now that that's over with (Mark Whalberg is the new human lead) this will finally "work," even if it still won't likely be the Transformers movie some are still hoping for.