Is this a troll question? Ofcourse btc is not bigger than usd that would be crazyness and btc would be way higher in price right now than it is.
Bitcoin is bit but not that big yet by far. We need more advertisement around the whole digital currency idea for it to get more big.
but it's not relevant for comparing the USD is having a pure value and bitcoin their value stands in usd, and bitcoins is exactly with the property.