Recruitment time vs town size

To help the A.I. help preserve it's populations and maybe also for playability

Town takes 4 turns to recruit a unit
Large town takes 3
Minor city takes 2
Large city rakes 1
and huge cities get a big fat bonus with the abilty to recruit 4 units per turn or if you just want to keep the pattern go for 2 units per turn.

Would make huge cities a much more valuable option and on top of that only being able to recruit a max of 1 percent of the population for a huge city while you can strip towns to nothing in a few turns is a bit detrimental to the A.I.

I think this idea could be quite good for the gameplay at times, though your system may restrain small towns too much. My main question is could it be done with the current engine and the access available to modders?

I believe it's hardcoded and it was more in reference to the exp pack.

You could do most of it in theory by making several copies of the existing units with different training times. You'd then edit the buildings file and make each of the recruitment buildings produce a different version of the unit, depending on the level of the building. Ie. Have 4 versions of hastati: Muster Field level barracks builds Hastasti 4 (4 turns to train), Militia Barracks builds Hastasti 3 (3 turns to train) etc.

It would be time consuming to set up, and there may be other implications. Personally I think it would a bit extreme to limit recruitment that way, but I suppose it has some merits - like slowing down initial expanse and the AI not stripping its settlements of people at the start.

I believe that it is just how hardcore you wants the game get that is all, for me I take about 24 hours or so to just edit few thing here and there.
Anyway if you want a more challenge game you don't needs to download mod from other, you could do it with a little background knowledge of what goes where.