Why does the Levi get more tithes of a field than the Kohen? Or in other words, why is Maaser Rishon so much larger than Trumah Gedolah?

Even if the Levi gave his Terumas Maaser to the Kohen that got the Terumah,
The Levi winds up with almost 9%,
While the Kohen usually only gets 3%

(Note: I know these aren't necessarily the exact figures, trying to simplify the case)

Is there a reason the Levi gets almost 3 times as much as the Kohen?