I understand, similar to how the leman model has a ginormous cannon and a super small turret, however that can at least be applied to it has super special materials or wat have u.
This is not the case with the imperial knight where it appears to just be the weapon and recoil with maybe a magazine behind the gunshield. The subcalibre thing could be plausible, but its stated to be a battlecannon which tends mean at least leman russ cannon size which means min of 120 mm cannon. So unless they have teleport technology then it can only fire 4 rounds without some kind of reload.
Just because its a model doesnt mean it at least cant be semi plausible with "future" tech and materials, i.e. similar to the Leman russ, or thunderbolt etc.