Almost all missions in XCF have more than 3 4 levels... Usually 10.
There are a few exceptions, but that seems to be in general a fact. There's certainly no combinatorial limitations on the capability of AI to plan its moves due to the increased volume.
Yes, I agree with your observations - if large, open maps were more common, it would be less of a "special agent" game, and more of a regular battle. A battle where pistols and shotguns have a marginal role, and artillery is the main source of DPS.
I am mostly trying to either keep maps only as big as required, or at least open.
In my experience, the style of battle is determined by the setting in which it occurs, and not the map size. On open maps, e.g. the ones with free-standing buildings, even with a smaller land size, the rockets and artillery are an essential source of damage, certainly so until the advent of really heavy armours (juggernaut+). The use of dynamite on early mansion missions is essentially a substitute for the mortars. Otherwise, there's a risk of incurring casualties, at the common 1:6 manpower ratio in favour of enemy. The usefulness of rockets is preserved in large indoor spaces, like bases or the white tower. The shrapnel rockets are more than adequate for most such settings. The Tt shrapnel rockets are a nice trophy upgrade. The mortars are essential in pre-juggertaun storms of battleships and terror ships.
When a battle moves inside the buildings, the machineguns and machine pistols play a more important role. I do find that a minigun works much better than even the BlackOps mag MP indoors. Still, the swords and grenades are an easy substitute for firearms in such settings, especially the psi blades. In fact, trophy White Tower ordnance is in general in sufficient quantity to supply the indoor assault teams.
What is really missing in the gameplay, is the access to early guided rockets. I think, there should be an option to field an early HE guided rocket (requiring either tritanium or aqua plastics), an early guided gas rocket (requiring either tritanium or aqua plastics), and slightly later, guided sonic rocket (damage about 180, with zrbite and aqua plastic requirements). I could write the config specs, but I'm not sure about producing the right graphics for those. Also, unguided gas rocket would be a good development, with the advent of gas grenades. Finally, a 200+ (say, 220 or 250, due to its larger size) damage guided blaster rocket would be nice to have, and it should be the only one to require elerium (just 1 unit, since it should be enough to initiate fusion reaction) (note that the alien versions could require more elerium to produce, due to the fact that aliens must have a much more ample supply of that substance, and could afford to sacrifice its quantity for compactness). A stun rocket, from a stun bomb, would be nice to have.
I think that one glaring omission from the game is the longer-range plasma sniper rifle.