2000-2400C/day sounds like a lot. I gain weight at 2000C per day except when I am out paddling or camping. I usually get by on about 1100C/day just sitting around the house. But, I agree this number varies wildly. I think that WWII germany showed that you could not starve someone at 2500C/day in a year doing hard labour, on average. They did some really terrible things to people...that was one of them. 1200C/day is bare subsistance level doing nothing but being a couch potatoe. MRE's were designed for bare subsistance. Leave it to the military to make sure you starve a little. It actually heightens awareness...
As far as lightweight food goes, you can get stuff that is very calorie dense. Parified butter or ghee is about 240-25C/oz. Olive oil is about the same. But, it is hard to eat large amounts of fats per day. I usually limit this to an ounce or two mixed in with other stuff. Fats/oils are good at calorie density and fat soluable vitamins.
Macedamia nuts, almonds, peanut butter rate close seconds for calories per ounce.
Pemmican is a mix of stuff and makes a good trail food at about 120C/oz. Lots of dehydrated foods are around 110-125C/oz. Dried sausage, salami, dried hot dogs (done in a microwave) supply lots of protiens, vitamins and other nutrients besides just fats and oils.
Good semi-sweet chocolate bars, honey, etc are down on calories a bit but still have lots of other nutrients that are hard to get otherwise. I avoid most sugars but carry some hard candy for quick energy.
Rice, pasta of any kind, flours (bisquick) etc are good for calories at around 100-110. Some protiens and other nutrients.
Cocoa mixes (skim milk, chocolate, sugars) are good. Fats and sugars. some other nutrients.
Generally, I pack about 130C/oz in about 18oz or about 2300-2400C/day. I have plenty of reserve, so I don't worry about loss of weight, I am rarely hungry but have been if I am out for a month or so.
Along with salt (a mix of lite salt and regular salt,) spices, and a few packets of gravy mixes/sauce mixes, this adds up to about 1.2pounds per day. Sometimes a bit more, sometimes a bit less. I eat late, usually close to bed time. This keeps me a bit warmer at night.