incorrect. This was actually a discussion on the forums once upon a time, and got heated enough to get a developer response. (Sometime in, what was it, 2001? 2002?)
The official 'word of god' from the line developers of the time, was that VTOLs, unlike Aerospace units, did not have to land to fire artillery.
I stand corrected on that point. And now see the Karnov being blown backwards Hollywood style when it fires the Thumper in the air. :D
There is few hover just because it is not so efficient - hovercraft's strength is its speed and it cannot have much weight, but artillery doesn't needs for speed and it needs some weight enough to carry the gun. But anyway nothing prevents hovers to equip an artillery.
Besides shoot and scoot to avoid counter battery fire? Or unassing if the enemy manages to get some fast movers behind you to make a run for the artillery?
Artillery isn't much use if you outrun its range, and the further out it is, the longer it takes fire to arrive. Hover and VTOL artillery is something you can assign to relatively fast moving units to give them artillery support that can keep up with their speed. Rather than requiring the commander to choose between leaving the artillery behind, or limiting his advance to its overall speed.
Hover artillery can also work as a response unit. Something to send if some part of your troops have a sudden need for more firesupport.
In terms of size, a 50 ton 8/12 hover with an ICE engine (Say, a stripped down Maxim frame and engine) is 24.5 tons, before armor or anything else is added. Besides Thumper and Arrow IV, that is just enough room to mount a Sniper and two tons of ammo. True the armor is going to be light, but it is doable.