In terms of food volume, you can reduce the volume of mountain house meals quite a bit by repackaging them into quart freezer bags.

For alternatives all you really need to look for is low water content. Water has no calories. Fats have the most caloric density, so foods with more fats are more calorie dense.

If you want total control look into getting a dehydrator and dehydrate the meals you normally eat to bring backpacking.

For lunch I love cheese and hard sausage with crackers or pita bread. Nut butters are also good. Hummus is actually fairly calorie dense too (even more dense if you get dehydrated hummus powder).