If people or things are massed, they stand or sit together in a large group, close and many. You hear it in rather fixed phrases: massed bands, a massed choir, massed ranks of soldiers, massed crowds. It often belongs to parades, big concerts, or army scenes — though actually it can just mean gathered thickly, as in massed clouds. It is the -ed form of the verb mass, “to come together in a mass.” It does not mean “huge” by itself. A massed group can be huge, but the word points to how they are packed, not to size alone.