I am not sure the Roman soldier’s diet was inferior. They ate mostly whole grains. When supply lines weren’t interrupted they seem to have been remarkably healthy. At least you seldom read about sickness being a big problem, even during sieges, whereas during the medieval and early modern periods it was always a race between starvation of those in the fort and the besieging army killing itself from filth.
The Romans may not have understood the germ theory of disease, but they were really good at practical sanitation. We didn’t climb back to their level till the middle of the 19th century.
There’s a lot we don’t have, which is accurate casulty records. The reason they were so effective is because if you poked a hole in somebody, they died. They had no sulfa, no effective antiseptic to prevent dying.
I’m not sure how many medical supplies the marines carry, but that stuff would be more valuable than their ammunition.