Stop acting so small. You are the universe in ecstatic motion.
Smallness, on this reading, is something we do rather than something we are. Rumi suggests that when we shrink ourselves by staying quiet, avoiding risk, or assuming our lives matter little, we are acting against our own nature. The human being is not a separate speck in the cosmos but the cosmos itself, moving with joy. The word "ecstatic" matters here, because it points to the Sufi world Rumi belonged to, where music, poetry, and the whirling dance were ways of dissolving the narrow self into something larger. Behind it lies the Sufi idea that the divine is present in all things, and that a person's deepest identity is bound up with that source.
A word of caution: this wording comes from modern English versions of Rumi, and it is hard to trace to one specific verse in his Persian poetry. It captures his spirit, but it may be a loose paraphrase.
In practice, imagine someone who always keeps a good idea to themselves in meetings. Taking the quote seriously might mean speaking up next time, not out of vanity but because their view belongs in the room.