Uh is it 20 food per grain? I think. Not sure.

seems to be 25 food per grain at the start of the game. there are yield modifiers in the XML file that defines the race (faction), i think your problem might be in those XML files. 

by default, the relevant part of the file should look like this:

<A_GoodieRating>1.0000</A_GoodieRating>
<A_GrainYield>1.0000</A_GrainYield>
<A_ManaRegeneration>1.0000</A_ManaRegeneration>
<A_MaterialsYield>1.0000</A_MaterialsYield>
<A_MaxArmySize>5.0000</A_MaxArmySize>
<A_NPCSpawnRating>2.0000</A_NPCSpawnRating>
<A_QuestingRating>5.0000</A_QuestingRating>
<A_Traits>4.0000</A_Traits>
<A_UnitUpgrades>100.0000</A_UnitUpgrades>

looks like a percentage based system with 1.0000 being the default value (i.e. 100%); i guess the faction you tries must have a value of 20 in the grain yield tag, this would result in a 2000% multiplier, i.e. 500 food per grain instead of 25. not sure how this might have happened, but at least taht would explain the number.

maybe that mod replaced the original faction definitions? not sure about the default factions, but custom factions aren't stored in Data\English but in My Documents\My Games\LegendaryHeroes\Race.

i suggest you find out where the faction definitions are and take a look at the values in the tags i posted above. if they are different from the default, either modify them back to default values manually or re-download the files.