Onions are over 75% water, Geoffy. They shrink to literally nothing. If you enjoy them, you should almost double the amount that a recipe calls for (if the onions are to be fried, which renders all the water).

Also, never salt fried onions until they're just about done cooking. Salt draws out water in vegetables (ever salt eggplant and wait an hour to see the residue? Yuck!).

If you salt the onions too soon, not only will they shrink too much, but the liquid that is rendered will steam the onions, instead of the oil or butter browning the onions. When onions steam, you get opaque, flavorless onions. When they brown, they get that dark, sweet, caramelized thing going on. Heaven on a bun, brother.