Looking at WW1 ration rates, I see that typically (and discounting water, which should definitely count against ration weight as well in some circumstances), the use of rations was reckoned to be 1.6kg per day per man on FS rations (preferred, 2 days supply carried in first line transport) and 0.75kg per man per day on Iron Rations (2 days carried on the man).
More importantly, the horses were provided with 10kg of Hay, Straw and Oats as a FS ration per day (2 days in first line supply), or 9.5kg of Oats as an Iron Ration - a horse not actually working could get by on forage and minimal supplementation, but this was seldom useful or profitable...
Enforcing fuel requirements for trucks, but allowing a free ride to Horse equipped units is a distortion.
I do note that the standard WW1 German first line supply was 4 ration days, while Cmd Ops penalises a unit cut off for 24 hours.
I also note that as Horse drawn units are using 'foot' movement rates, it allows free and easy movement of heavy artillery across 'uncrossable' streams. In most ways, horse drawn artillery was less mobile than a motorised equivalent (possible exception deep mud, or when there is a fuel crisis), and the gun equipment and ammunition is certainly no handier.