Marching was already mentioned. But you forget an occupation, ancient world soldiers' were exposed to in large amounts.
Let your soldiers build stuff. Bridges, walls, camps, roads. Some of that they'd need for the campaign. Some not. Building in times before steam or internal combustion engines means a lot of physical work. Probably, all the woodwork would need to be sourced by the army, which means even more lumberjacks. All these are physically exhausting activities, warranting large amount of intake calories.
Make the soldiers accelerates, i.e., the modern body type and size – large men need more calories. Extreme, professional sports or extreme weather conditions would require to reach such intakes.
That's the gist, now let's check the harmony with algebra.
I've dug up a formula on needed calories based on body parameters and activity:
$$(88.36 + (13.4 w) + (4.8 h) – (5.7 a)) \; c,$$
- $w$ is weight, in kg
- $h$ is body height, in cm
- $a$ is age, in years
- $c$ is the coefficient for workload type, e.g., 1.2 for no load and 1.9 for heavy load.
Now, let's search for a good parameters. Let's say, the whole army is build from 190 cm tall, 20 years old guys. The top margin of healthy BMI says, they would weight 90 kg. (Let's pretend, it all muscles, not fat.)
These values yield something shy of 4000 calories. Even if we force then to inhuman, bone-crunching work, and raise $c$ to 2.2, that's still only 4600 calories. Too little.
(Notice than while praxis has seen the consumption of the desired amounts of energy, that's not what regular mortals' dietologist would approve of for regular life. So, either make their life really hard, per above. Or – keep reading.)
A normal human does not suffice. So, let's go Warhammer.
Let's take a three meter tall space-marine breed. Scaling up 1,5 m to 3 m height scales up the weight 8 times (because of square-cube law). Let's say, that's 400 kg.
Per above formula, those guys would need more than 8000 calories even idling, and 12000 when heavy working.
So, you either need to work your soldiers to the bone (by forcing them to do hard manual work all the time, by marching insane distances, or by conquering Antartica), or you need larger soldiers.