Interesting question. Myself, I tend to avoid the VERY-long/light khukuris, but also the super-heavy choppers. I'm thinking about why, as I write this.
Here's what I come up with. I live in Arizona, the southern and central parts of which are covered by the Sonoran Desert, the northern parts of which have everything from high desert to ponderosa pine forest to (on a couple of the highest peaks) alpine permafrost tundra. When I'm getting a knife for camping, I end up taking all of this into account.
What we don't have a lot of in my desert (the Sonoran Desert) is lots of big trees that you'll want to be chopping every day. For this reason, the heaviest, longest choppers I tend to leave at home. That said, the wood we DO have here in the actual desert tends to be VERY hard--mesquite I chop a lot of, and, in the city, olive and citrus wood. These can be like rock, sometimes. One also sees a fair bit of desert ironwood around, and if one were out and needing to make wooden things in a hurry, that'd be useful to have. What's this mean? Well, an 18-inch big chopper gets to be a little much, heaviness-wise--but I do routinely find myself using standard-length (16.5-inch) chiruwa ang kholas, just because they do pretty well against extremely hard wood in fairly-narrow-diameter pieces.
I also use a BAS a good deal--it's smaller than the others, more easily carried, and actually takes care of most of the chopping tasks I would likely run across.
I guess I don't end up using the over-16.5-inch models as much as I do the 15-16.5-inch ones, just as a matter of what I actually use. But I do find that the often-hard wood in the desert makes me a little more comfortable with a somewhat-stout blade, when I'm actually chopping. And I've got to admit that I've never had any kind of failure using a thinner sirupati or chitlangi or Gelbu special/Udhaipur variety, so I may just be overcautious.
Deserts vary; the vegetation differs remarkably from desert to desert, and you'll find that some have almost nothing to cut, while others are full of stuff that cries out for a large knife. Even in the Sonoran Desert, there are a lot of different micro-regions (or "biomes"). These range from stark, sand-dune desert (e.g., near Yuma, Arizona) to quasi-jungle (far down into Mexico). I'd give some thought to what you think you're likeliest to want to cut. The tall cacti you see a lot of around here (saguaros), there's not much of a reason to cut. The fruit, you get off of those with a couple of long poles tied together--and it's usually not hard to find a few of the soft, woody ribs from a dead and dried-up saguaro anywhere where there are a bunch of live ones standing. Prickly pears can be eaten, as can the buds of the wickedly-spiny chollas (and some cholla buds are about the best available source of calcium out here, so one could imagine wanting to tangle a bit with those plants.) Another potential food source that might make you glad to have a long-bladed knife is the agave (See, e.g.,
http://www.desertmuseumdigitallibrary.org/public/detail.php?id=ASDM01585&sp=Agave parryi parryi ,
http://www.huntington.org/BotanicalDiv/ISI2004/isi/2004-07.html ). Some species of agaves were absolute staples of the native peoples here, and a great source of carbohydrates. The natives used to harvest them, I read, with implements shaped kind of like large wooden chisels, pounded on to drive the edge under the "head" of the plant and sever it from the root. You would do this right before it sent up its flower stalk, which would be when the plant had stored up large quantities of sugars, etc. I'm told you look for agaves whose center leaves are closely-spaced and a bit shorter than the surrounding leaves--easiest to do if you can view a large number of them and compare, as it's a little subtle. One needs to cut off the many leaves guarding the central "cabeza" or "head", as they are tipped with needle-like spines and the edges of the leaves are likewise armed with wicked, usually curved, spines. The leaves themselves are full of tough, usable fiber (which makes them useful in their own right, as a survival material--but also means you'll really want a sharp knife, and probably a pretty long one, to get through them easily.) The natives would gather huge numbers of these "heads", and cook them for a couple of days in huge, stone-lined pits full of hot rocks and agave leaves, covered with dirt. If you do it just right, the insides of the heads turn to this sweet, vaguely-tequila-flavored substance that is a great source of sugars, etc. If you do it insufficiently or pick the wrong kind of agave, you can end up with something mouth-blistering and inedible--or, if you cook them too long, you can end up with a mass of charcoal. But if you can do it right, it's a great resource. (FYI, the natives used to rely on these so much that one sub-tribe of the Apaches--the "Mescaleros"--was named for harvesting this. Around Arizona, ethnobotanists have found that some species of agave, apparently much-cultivated before Columbus, appear in most cases to be natural clones from a single ancestor plant, having been deliberately distributed by paleo-Indians. One can still find places where these ancient "gardens" still grow, 500 years or more after the civilizations that planted them died out or moved on. Sometimes, I read, one can find the ulu-blade-like stone chopping tools left nearby on rocks; Wendy Hodgson's book on Food Plants of the Sonoran Desert has a picture of such a knife--looking basically like a flat stone, shaped kind of like an ulu blade--that she says was found on a rock near an agave planting--probably 500 years after its maker passed on.)
Anyway--back to khukuris. It seems to me that just about any khukuri would be well-suited for agave harvesting; my personal preference tends toward the 10- or 11-inch-bladed ones that can handle some pretty tough wood. I guess that unless I were solely using it for agave harvesting, I wouldn't see much use for one that was very long and thin. But, again, your mileage may vary--just as your desert may!
Have fun!
Edited to add: It occurs to me that you may be shopping for a khukuri for a soldier serving over in Iraq or thereabouts. I've got to plead ignorance on what THAT desert is like, and what you're likely to find there that you'd like to chop (well, besides bad guys!) If that's what you're after, maybe someone who's had a little more time in that particular patch of sand can tell us both something.