Here's a quote form Dunnigan's old book "How to Make War" (2003, p. 124) that - while old - still largely applies:
Depending on the type of division and nationality, infantry comprises 8-30 percent of division strength, tank crews 1-10 percent, and artillery (including antiaircraft and antitank weapons) 6-12 percent. Combat troops comprise an even smaller potion of non-divisional forces, something like 5-10 percent. Since combat divisions account for 20-50 percent of army manpower, combat troops comprise only 10-25 percent of all personnel. In all armies, combat support troops are very much the majority.
Elsewhere he wrote that infantry comprises about 5 percent of a Western army's personnel strength only.
There was a move away from division-centric to brigade-centric force structures since, but the overall picture remained about the same. Thus let's take Dunnigan's summary above as a basis for some thoughts:
Some support efforts have a fixed size (equivalent to fixed costs in economics); area air defence by a SAMP/T battery does not scale with the combat troops in the shielded area, except that it is either provided or not. Bridging requires about the same effort, regardless of whether one or ten tank battalions are going to cross that bridge. Load carrying capacity (MLC 60, MLC 80 etc.) doesn't affect personnel demands much either. The width of a bridged river on the other hand changes personnel requirements slightly (more pontoons = more lorries to carry them = more engineers for driving and installation). Electronic warfare is similar; either you have those jammers to blind a JSTARS-like aircraft 300 km away or you don't. Either you have the receivers in position to triangulate radios in the area or you don't - there's no proportionality with the quantity of combat troops in the area.
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A look at tooth to tail ratio or a listing of percentages as Dunnigan provided implies a relationship between combat troops and combat support (fashionably: "sustainment") troops that's following a stable causality. Now I pointed out that sometimes the terrain features instead of the combat troops are the influencing variables. Long-time readers of D&F may probably sense where this is heading:
The 'book' needs to be rewritten and reconsidered for low force densities in state vs. state or alliance vs. alliance (a.k.a. "conventional") warfare.
A 100 x 100 km area may see (among other forces) five manoeuvre battalions, one area air defence battery, one independent bridging company, one field hospital with MedEvac helicopters, one battery with 100 km range artillery missiles and one jammer against radar aircraft.
What happens with the support elements if there's but one brigade in action in the area? Will there be the same amount of one-or-nothing support elements or none? Does this problem lead to an even smaller share of combat personnel in an army or to the opposite, a cutting down of combat (service) support of the one-or-nothing nature?
Second challenge; what about small allied powers' contributions? They do likely not (and should not) have the same assumed-to-be optimal structure in miniature. Shouldn't the larger armies with focus on collective defence compensate for this by having even more than optimal shares of support troops in order to reach the optimal mix in a collective deterrence and defence effort?
Another challenge; with only 10% combat troops an opposing force needs to cause less than 5% casualties to break all of our ground forces and to force them into a general retreat. Maybe 2% if the offensive is limited to one region. We sure need to have personnel reserves for the refreshing (refilling) of combat units, but (assuming these reserves are not in the support units) why wouldn't we employ these reserves right from the start if this leads to a less than proportional growth of support requirements, in low force density scenarios more pronounced than in a long time?
In the end I advocate a close look at the optimum mix for low force density scenarios, while keeping in mind that over time the arrival of reinforcements (even from different countries) will change the structure and the force density in the theatre of war - and thus also the optimum structure.
There's a pretty strong case for thorough research, experimentation and alliance-wide coordination in order to avoid wasteful spending and a waste of the soldiers' time with surplus capacities.