The photo gives the impression that the wheels are about chest-high against a standing man, which doesn't accord with the Hat model, so perhaps the wheels of the Hat model are slightly over-size – and yet the model as a whole looks proportional and in scale with 1/72 figures.
'If the gun is under-scale, it's not by much.'
It's not the proportions of the entire STP model that are doubtful Mark, but merely the diameter of the wheels.
Compare the relationship between the height of the wheel and the kneeling gunner second from left in the photo with that of the wheel of the STP model and the kneeling figures. In the latter case the top of the wheel appears to be on the same plane as the top of the figures' heads, whilst in the photo the wheel is markedly taller than the kneeling gunner.
In the photo the third figure from the left appears to be seated on the gun. Note that he's significantly elevated above the kneeling gunner, whereas the STP seated figures are only slightly higher than the kneeling figures; another hint that the model's wheels are too small.
Certainly the STP wheel is about chest-high against the standing officer figure, but compare the height difference between the kneeling and standing gunners in the photo to the height relationship between their STP counterparts. The real kneeling gunner is about waist-high against any of the standing gunners, whilst the kneeling STP figures are chest-high against their officer, which therefore appears to be severely undersized. If it was in proportion with the kneeling figures the wheel would only be about waist-high against it.
I'm off to have a Bex and a good lie down.*
*Redundant colonial expression.