The reason he looks squat is due to the fact he needs to scale in relation to the current masterpiece figures. In the cartoon Ultra Magnus is a similar size to Optimus in robot mode even though Optimus is just a cab and Magnus is the trailer and cab. Since we don't have mass shifting in real life with Transformers you need to put that extra mass somewhere and the most obvious place is making him squat and stocky. The resulting figure is still larger compared to MP10 but he doesn't completely dwarf him like citizen stack. Scale for phase 2 of the masterpiece line is extremely important in my opinion.

You're also forgetting about cost it has been noted that the lack of the white prime has made him much cheaper. With the added complexity of his trailer would you or everyone else be fine with paying over 300 USD for a white prime brick that needs to parts form? This Magnus is going to be half the price and considering what we're going to get it's a steal. Now even though he isn't out yet I can't say if he'll be worth that but I'm guessing he is going to be great considering Takara's past MP's and how he looks right now I'm quite pleased with how they went with him.