You have trained in another art, put some knowledge into that cup, but now in coming to a new art you have doubled or tripled the size of your new cup and now there is just about enough to cover the bottom of this new cup.
Personally, I fail to see how, in one day deciding to study a new art I have doubled the size of the proverbial cup... I agree however, that the cup grows, as one grows over their spectrum of experiences. But my experience shows me that this growth is reactive and not proactive. Experience opens the opportunity to increase the volume of stuff present, and also gives guidance as to what ratios this volume should contain.
The proverb as it stands does nothing more than tell people to not let experience hinder new learning. Which it does. It is only in higher stages of understanding that we can compare and synthesize.