I call BS on those measurements, for the record. Whether it'll matter is unlikely.
Looking at the image, with the knowledge that the cannons are 120mm, and using a rough approximation of the barrels relative width at the end, which should be about 120mm, or 12 cm, I found that the Guntank is more likely to be about 2.5 meters tall, not counting the antenna.
What was scary was that when I tried to found the height of the antenna and account for it, they turned out to be exactly 3.6 meters tall. Which was creepy.
In any case, that would be a waste of height if it was really 15 meters tall, particularly for the guntank. Not to mention risky for the occupants, since they can't obviously can't take cover anymore. But I mean; how many tons of material, for just 2 120mm cannons? And it's meant to operate as an artillery piece anyway? Lacking the speed and maneuverability the bipeds supposedly get which makes up for being a stupendously large target, and the melee weaponry that would make it's 'hands' of some sort of use?
If the Guntank is piloted in the traditional way similar to Gundam suits, I can see zero reason for them designing it in such a manner, besides trying to look cool.
Goddarn military designers these days, I tell you what...
EDIT: *facepalm* Meanwhile the guns themselves have barrels approximately as long as the Guntank is tall...3.6 meter long barrels...that's actually pretty short, compared to real life tanks(Abrams is about 5 meters long, and the tank itself is 2.4 meters tall).
So Guntank is extremely efficient, if it's small, being tiny and bearing a humanlike shape to accomodate for the operator inside, though it would be useful mostly for urban combat as anti-armor and tank hunter roles. Also it would probably knock itself over with the recoil but whatevs.
SECONDEDIT: While the height's probably accurate, the barrel length's probably a bit longer than what I calculated since it's a 3/4 view.