The only qualm I have about this thread's proposal at all, and I've mentioned this somewhere before, is that I believe that MUCH more land should be required for farming, and I mean on par with what human farms are like in the game and would be required in real life. To put it in other words, not only do I think people need to eat around 5 times more than they actually do in the game, I think the rate of production of food should be reduced per tile of farmland. This would open the game up to some very fun, not to mention realistic, challenges, such as having to manage and protect lands outside of the immediate embark area (unless farming is done extensively underground under the embark area).
Well, part of what I want to do is make it so that
if you use the techniques of that era, then you will be correct - it takes up a huge amount of space.
However, part of what you can do is have full irrigation, more complex crop rotation schedules, you can balance your fertilizer loads more carefully, take advantage of polycultures and aquaculture and grow your cattle's fodder on salty/swampy ground that isn't suitable for standard agriculture, use "natural pesticides" like fostering spider colonies and bat colonies to keep pests down and otherwise have tricks that give you an edge if you know how to use them.
Simply tilling everything into one giant patch of wheat for a single staple crop was what they did largely because wheat was the most easily traded crop, and the easiest to store for long periods of time. Wheat actually takes more land to feed the same amount of people than growing vegetables does, and many medieval farmers would actually subsist on smaller vegetable gardens and a small orchard of fruit trees for much of the year, and only use bread to get through the winter, along with the few winter crops they could still grow (like onions or turnips or other root vegetables, and potatoes once they were brought back from South America).
Part of the reason the Muslim world was capable of exploding in population, wealth, and power during the Middle Ages was that they had more complex land management systems, public works including irrigation that Christian Europe didn't have, and could feed a much greater populace with less farmers.
Even then, it's worth noting that medieval farmers were only part-time workers - they really only worked hard during the plowing and the harvest. They did almost literally nothing during the Winter. Farmers were known to "waste away" during the Winter, not eating, not going outside, not doing anything but staying near a fire, and losing 50+ pounds of muscle during the Winter months.
The ancient Egyptians, in fact, had farmers, not slaves, build their pyramids - when the Nile flooded, and farmers had nothing better to do with their time, the Pharaohs came up with public works projects for them to work on in the meantime in order to prevent 2/3rds of their populace from doing nothing but getting drunk and causing trouble for three months at a time.
Also likewise, people will tell you that Asian rice paddy farming is the most land-efficient means of food production, which is true, but it's also the most labor-intensive form of food production. An ancient family working with wood or stone tools in a field of wheat could work 3 acres of wheat farming. Before the Industrial Revolution, with metal tools and better irrigation techniques, this rose to 10 acres. Rice Paddies? 1 acre per family straight up until modern mechanized farming. Sure, you get 3 harvests per year in tropical regions, but rice paddies are extremely maintenance-intensive.
Now, how many acres can a fruit orchard handle? If you have cattle-drivers in mostly arid semi-desert grasslands how many acres can a small number of cowboys cover with a herd of cattle?
Land area and labor required to produce a given amount of food are not necessarily proportional to one another.
I'm presuming players will probably want to graduate up to the Muslim levels of land management, where it can still be realistic, but not take up 90% of your fortress farming.
EDIT:
Or, to say this a different way, I can understand why you would start from a place of saying "this should take X amount more space to run a farm", because the farms we have now are very much a matter of every individual crop being basically the same, however, that's not the sort of system I am trying to make.
I am making an assumption from the start that real-life farms would never expect to get maximum yield consistently. Nor should the player if they're smart, but that's another matter. Let's assume for a second that we could have gotten 3 times as much actual grain from a wheat field than historical farmers did simply through the use of more intensive fertilization and irrigation so that the crops didn't wither without rain.
There's also the fact that basically all crops in the game now are fairly similar in how they grow, and growing more crops in this game is generally a function of how much land you are using for agriculture. Now, let's talk about how we have crops that have different nutrient consumption rates, and how we are limited in our use of fertilizers. Hypothetically, we could grow much more land-efficient crops that take more labor per acre to grow those crops.
Alternately, it takes very little effort to just set up a nest box or farmer's workshop and have a pasture for chickens or cows over "wild" land that you don't till or work, and let be fertilized naturally by the chicken and cow manure that falls out of them naturally as they go. That takes little labor, but a lot of land if you aren't going to manually optimize the fields. Also likewise, you can set up fruit orchards, where the apple trees you harvest for fruit still need care and maintenance, and definitely needs labor to harvest, but obviously not as much as plowing a field and constantly having to refertilize the soils.
You have choices between how much land you use, how much labor you use, and how much of your fertilizers you are going to use to get the most crops based upon what resources you are most feeling the squeeze on at the time. The optimal choice depends on your given circumstance.