BICEP2 is a cold detector. To see B-Modes on gravitational waves from the Inflation Era, one needs low temperatures. Noise would erase the subtle polarization signal one is trying to get. These polarization arrows are a thousandth, or less, than those from the E-Modes, say. More area, and less temperature, to collect enough polarized photons with the tell-tale signature, of a weak gravitational wave. At the moment of Inflation, the magnitude of those gravity waves was huge, but we only get weak echoes, 13.8 billion years later, that is why, the collecting area has to increase by a factor of ten.
Less temperature means less Information, as Professor Luis Herrera, has recently proposed; so one needs a bigger collecting area to compensate for the lowering of the temperature, which in itself is necessary to reduce the noise. Signal to Noise, was bigger than 1, so we saw the signal.
The measured tensor to scalar ratio is: