The other significant difference is the Thard River valley; it's very rich, and Lake Benath is also an important resource for the production of food, neither of which Rome had access to.
... errr, no it had a funny thing called the Mediterranean!
That aside, an interesting statistic to throw into the calculations. According to Astill & Grant in "The Countryside of Mediaeval England" p14
A quarter [of wheat is] enough grain to keep an adult alive for 1 year
FYI, a quarter is 8 bushels. Now, I have looked briefly at the bushels/acre production of Harn, and based on a yield rate of 72d per acre (Manor 29) and a price of 8d a bushel (HarnPlayer 18 ), you get a yield of 9 bushels per acre. Therefore, every 8 acres could support 9 adults. An average of 320 acres of an estate (just an illustrative number) would support 360 adults. If you assume that a household is 5 people - which does fit with some Harn numbers - two adults and three children, and that the children eat 2/3rds the adults, a household needs 4 adult-years of grain. Based on 360 adult-years, the 320 acres would support 90 households.
Now Astill and Grant only say that a quarter supports 1 adult, but not how. However, a quarter of grain weighs about 480lb, or about 400lb of flour. That translates into about 1lb of flour per day or a loaf of bread per adult per day.
To me, that supports the adult need only, and not livestock.
Here's why: Using a standard recipe for a cottage loaf, it would translate to about 2250 callories and 6g or saturated fat /60g total fat. A man should get 2500 calories and 95g of fat, a woman 2000 calories and 70g of fat (based on UK recommended diet). Average that you get 2250 calories and 82.5g of fat. So the loaf meets the average daily calorific amount, and is slightly low on the fat - add a bit of butter and you get the full amount. Thus the amount checks out, although I assumed a 100% efficiency in milling, whereas the true number is closer to 70-75%; it would imply that A&G looked at just the milled number, not the actual produce, but they were looking at the final amount needed. Applying that number back to Harn, the 320 acres now only support about 63 - 67 households. Call it 64 households, and you get 2 households per 10 acres of cropland.
Now, I'm not a farmer, so I don't know if the acreage yield checks out - that is the value that Harn would seem to produce - however, a North Dakota University study found that their yield was about 25 bushels per acre, but that was using modern farming techniques. Organic farming techniques yield about 20% less than "modern" techniques - according to a Swiss study and reported May 31, 2002 in the LA Times.
That means current organic techniques would yield about 20 bushels / acre today. Harn comes in at about half, but the labourer of Harn would not have the machinery of today (natch) and the fields would have a high mix of meadow flowers which would reduce yield. The amount on Harn to me seems a bit low, but not too badly.
So there ya go: 10 acres produces 90 bushels of wheat that translates to enough to support about 2 households. Assuming meadow supports the animal, woodlands the pigs - well that is another calculation!