The title is relatively self-explanatory, here – the poem is singularly focused on a poppy, and so "Big Poppy" seems like an apt title. But why not just "Poppy" or maybe "The Poppy"? In keeping with the tone of the rest of the poem, everything about this piece wants to emphasize just how incredibly large this flower is – in a literal sense, and also in a metaphorical one. So the title, then, by the time we've read through the poem, becomes more than just "a flower of unusual size." It's also "big" in the sense that it's conceptually large, philosophically large; it becomes a world of its own right before our eyes. We mean "big" like "meaning of life" big, like "the birds and the bees talk" big.