Depending on your dexterity and finger strength, perhaps you don't need so much key, but think about being able to both hold the key firmly and get enough leverage on it to twist the key in the door locks and/or ignition. Also, if you live in an area that gets cold, larger keys are easier to use with gloves on.

The downside to this is that I've heard of people bending keys in frozen locks because they had a little too much easy leverage and tried to force it. LOL