I've never hiked for more than 18 days, so weight loss was usually covered by excess body weight at the start. Actually, it is one of the reasons I do this stuff.
If you don't have a lot of on-body reserves, you just have to pack more. I wouldn't mind having that problem, because the extra 15 pounds I carry at the start of the hike are in addition to the food I'm eating, while if you carry that extra food to cover your nutrition needs 100% would drop rapidly after each food pickup, giving you an overall lower average trail weight.
The freeze dried stuff really is bulky and I ran into that problem last summer when we had one Bearicade Expedition canister for me and my two 10-year-olds. That thing gets filled very quickly if you go with the Mountain House stuff. We used a lot of angel hair noodles and other flat packing asian noodles or rice to bring more stomach-filling food that doesn't need a lot of space in the canister. We still didn't get more than 5 days of food into the canister, which lead us to get a second Bearicade for this summer's trip.
My favorite calorie packers that don't use up a lot of space are
Gatorade powder (the real sugar stuff) - also makes that whole water drinking much more enjoyable and seems to be my best recipe for those low energy moments on the steep switchbacks
gummi bears - lots of sugar and fast conversion to energy
nuts - fats, some metals
Cliff bars - highlight of the day for the kids
peanut butter and jelly - fat and sugar
Scho-Ka-Kola - hard to find in the US - it's a caffeinated dark chocloate German pilots in WW2 already used as emergecny food. Comes in a tin can (leave that at home). Very much an emergency energy booster and doesn't seem to go liquid in the summer heat like other chocolate. A whopping $8 a tin here:
http://www.germandeli.com/4001743034013.html (On the back it says 4 pieces are equal to a strong Espresso, 16 pieces in the tin)
None of that really fills your belly, though - high fiber food does. We bring a lot of home-made cereal that contains rolled oats, dried fruits, some sugar, granola, and mix that with real dried milk ("milkman" brand - not that fat free stuff you get everywhere). For the main meal of the day it's always something with fast cooking rice or noodles which bulk up nicely with water and fill your belly. The only advantage of those freeze-dried foods is that you can eat them without cooking, so if you run out of fuel or your stove fails, it's still ok. I usually pack one or two of those meals for a 5 day load, just in case.
Last summer I lost 18 pounds over 200 miles, carrying about 35-45lbs, while the kids did ok and maybe lost 5 lbs max carrying 12-15 lbs packs. We ate similar portions each meal, so I guess I packed in my extra food on my hips :-)
4 months after the hike, my weight was right back to where it was before... ready for 2009 I guess.
The thing about that weight loss is htat once your body weight drops down low enough, your energy expenditure also drops. After about 10 days on the trail, the weight loss usually slows down and the body also adapts in how it processes the food it does get (I am guessing here, but I think the way food is digested becomes more efficient during long term hiking, because the overall expenditure doesn't really drop that much on a 180 lbs adult with a 50 lbs pack if he drops 15 lbs of that in body weight).
How do people do the PCT? They don't eat that much more, all lose a lot of weight, but once you reach a level of fitness that matches the activity, I assume your energy conversion into distance and altitude becomes much more efficient. Those people are not starved to death when they finish, nor do they carry huge amounts of food. Plus, when I read their reports, I frequently read of the "trips to town" where they completely binge on calories to tank up again. Perhaps you should just plan on a 2 day layover at the pizza factory in Lone Pine (meat lovers special... 'nuff said)