There is a very good reason why Citizen Stack is so wildly out of scale with MP-10.
It is because KFC used the following scale chart image for size reference when they made the figure:
HOWEVER, there's one little problem with this particular scale chart: It doesn't exist. Unlike the official one that Takara's been using for sizing the official MPs, the scale chart seen in the above image that KFC used for Citizen Stack is fanmade.
All who claim that it is official claim that it comes from the
Transformers: The Ark book, which is the book that has the official scale chart that Takara uses. However, many people (including myself) own the book and have confirmed that the scale chart found within its pages contains no image of Ultra Magnus towering over Optimus/Rodimus/Grimlock as seen in the above image. The only chart to be found in the book that features Ultra Magnus and Optimus together is one that has Magnus only a head taller than Optimus:
Full size
There is no way KFC could have gotten the scale chart in the first image above from this book without someone first editing it. Therefore, it has to be fanmade.
Now, as to why someone would go to the trouble of deliberately making their own scale chart to have Ultra Magnus be so ridiculously tall compared to how tall he is in the real scale chart is not known, nor does it even make much sense. I can only imagine that someone believed or wanted to make Ultra Magnus be bigger than he really is, and so made their own fake scale chart to justify their fantasies. It's far-fetched, but crazier things have happened.
All that said, though, the official MP-22 Ultra Magnus is still a bit taller than he ought to be, being a couple heads taller than MP-10 instead of only one head taller like in the official scale chart. But I guess Takara tried their best to match the scale chart as close as they could, but could only get so close without magically losing the existing mass of the vehicle mode in robot mode. At least it's not nearly as massively off in size as Citizen Stack is: