Consider the definition for full (Source):
full [foo l]
adjective, fuller, fullest.
completely filled; containing all that can be held; filled to utmost capacity: a full cup.
complete; entire; maximum: a full supply of food for a three-day hike.
[More definitions exist, but they all seem to be variations on being at the maximum. If there's an important difference, I don't see it.]
Note that there exists the compartive form fuller and the superlative form fullest.
But how is this possible? By definition, something that is full is at its maximum. So how can one thing be fuller, or more at its maximum, than another thing also at its maximum? And how can one thing in a group be the fullest, or the most at its maximum, than other things also at their maximum?
In short, how can something possibly be "fuller" or "fullest"?
(For what it's worth, I've seen these words in the wild: the local school district has the motto "All students will achieve their fullest potential". Students and teachers alike are unsure of whether or not that makes any sense.)