The $19.95 novelty ones are a toy and a joke. There are high end ones that are serious weapons. Many crossbows have the prod [the bow part] attached by bolts that can easily be removed. This can make a crossbow either pistol or full sized more compact for carrying. While bolts are reusable or easily fashioned the critical part is the string. Because it drags along the body of the bow it wears quickly. This would behoove you to include a backup or maybe several.

Personally I prefer a long bow. The limitations are two fold. One you need both arm fully functional to use. So a crossbow might be better in a real survival situation where one might be injured. The second is that it takes a great deal more time and effort to learn and to become proficient with a long bow. Many of the newer non-compound ones can be broken down into the 2 limbs and center section. For a very compact package. While the string does not wear as quickly it is still the most likely thing to break. Carry a spare.

Slingshots are smaller and easy to carry. Their pluses and minuses have been covered.

There is also the sling of David and Goliath fame. Before you think these might not be effective. Remember these things were a weapon of war. A trained slinger could throw a chicken egg sized rock or lead pellet and drop a man at upwards of a hundred yards. They make a package that is about half the size of a fist. But like the bow it takes a lot of practice to be good. If you try one don't expect bulls eyes right off the bat. David while tending the flocks had many hours to wile away. Throwing rocks with a sling was probably a way to entertain himself.

Another compact weapon I carry is a bola. Yes to be really effective they should weigh a couple of pounds. But there is a trick to make them light and easy to carry. This is to replace the weights with cloth drawstring bags. The lines and the other bags can be stuffed in one of the bags. When you need to use simple unpack and then fill the bags with whatever is at hand. This could be dirt, sand, rocks and even snow.