What really should happen is that some of the map data isn't tracked per tile, but that each tile has 1 byte with which to reference "other data about this tile." It'll increase the overall memory useage of a map by that 6.5 MB, but you could offload the fluid bits and a few other "rare to have in every tile" bits into a separate memory location. Then only the tiles for which this data needs to be stores is stored. 1 byte for a tile to track soil quality? With 1000 farm tiles? Why that's only another kb of map info! Eight new fluid types? Why that's only 3 more bits per tile (and only for the tiles that have any--non-zero--fluid amount!)
Any tile without any "extra info" would simply have a null address, still using 8 bits of space, but it allows far greater expansion of increasing map data without significantly altering the overall map data size.
Keep in mind we are dealing with multiple variables. I would say three as an absolute minimum for soil quality if we are to have the sort of system where crop rotation would make sense, plus water, and each of these would need around a byte for the sort of things we'd like to do with them.
Also, I did some research of my own, and to get back to the number of tiles it takes to feed a dwarf, based upon my own
estimates of the size of a tile, came up with some numbers of my own.
Apparently, the minimum idealized amount of land that a purely vegan average-sized human needs to feed himself for a year is 3,000 square feet. (This is according to some random dude named Jason Bradford, who is, incidentally, one of those guys who says we should go vegan because it involves less land use, so he may just have some goods to sell on this account. Take my lazy Internet research with a grain of salt.) I calculated out the size of a tile at 100 square feet. There are other factors to play in, like the winter growing season of DF, but we can use this as a quick-and-dirty calculation to arrive at the idea that humans take 30 tiles of farm per person. (Dwarves are 6/7ths the mass of humans, and, assuming equivalent metabolisms, would presumably eat 6/7ths as much. That reduces it down to roughly 26 for dwarves.) Mutiplied by the number of dwarves in a fort, and you get 2,600 to 5,200 tiles per dwarf
if we wanted to be completely realistic. This is also talking about farms which are always in the same, presumably close-to-ideal conditions, meaning that drops in soil quality would require a matching increase in land to compensate. This is also not taking the skill of the farmers into account, when farmer skill makes a MASSIVE difference in crop yield in the current version. (I honestly don't want to be that realistic... especially since, realistically, cities or fortresses IMPORTED their food from a very large, open, and completely undefended countriside where most peasants lived at barely above subsistance agriculture. We simply wouldn't have a self-sufficient fortress if we were being that completely realistic.)
Now, we could theoretically start talking about the numbers that would be required to be jammed into the machine for it to spit out our target. If we want 26 tiles to feed 1 dwarf, who eats 8 times a year, we are talking about 1 tile producing roughly 0.31 food per year (this is assuming we are making 1 food = what a dwarf eats per sitting, which is something I've been arguing against in things like the
reasonable foods and
volume and mass threads). We should presume that, even with longer growing seasons, we should be at least theoretically able to harvest twice a year, and considering as it is theoretically possible in the real world to harvest some crops three times a year, even without winter farming, it really should be something higher, anyway. This brings us to about 0.16 food per tile per harvest (or 1 food per 6 tiles), in "normal" soil conditions, with "normal" farmers, with a "normal" 2 harvests per year.
Again, I don't particularly support that, but those are the numbers.