Something that's full holds as much as it can. If your glass is full of root beer, it's up the brim — no more root beer will fit inside it.
When a trash bag is full, it's time to take it outside, and when your mouth is full of cake, you'd better swallow it before you take another bite. When you're full of pizza, you can't eat any more, but when you're full of self-confidence, you have plenty of that quality. When the moon is full, it appears as large and round as possible. When your skirt, hair, or body are full, they're wide or ample.