Gizmodo has a pretty detailed explanation of Why It'll Take New Horizons 16 Months to Send Us This Week's Data.
Running on a mere two to 10 watts of power—roughly as much as a nightlight— each of New Horizon’s seven state-of-the-art scientific instruments is currently busy collecting a deluge of data on the surface composition, atmosphere, and geologic features of Pluto, its moon Charon, and its four smaller moons. This data is being sent to one of two onboard, solid-state, 8 gigabyte memory banks. From there, the spacecraft’s main processor—a radiation-proof 12 megahertz Mongoose V—compresses, reformats, sorts and stores the data on a recorder, which NASA likens to a flash memory card for a digital camera. Once stored and formatted, the precious science and telemetry (aka housekeeping) data is ready for transmission to Earth—it’s being sent in compressed format now, and will be sent in a lossless format later on.
New Horizons communicates with the Earth through a series of four dish antenna. For key scientific data, it’s primarily making use of a large (2.1 meter-wide), high-gain antenna. But the high gain beam is only 0.3 degrees wide, means New Horizons must be pointing straight at the Earth in order for us to receive its signal. That’s why the craft’s comm system also includes a wider-beam (4 degree) medium gain disk, which it can use as a backup in cases when pointing might not be as accurate. The craft’s comm system also includes two broad-beam, low-gain antennas, which were used at the mission’s outset for near-Earth communications but are largely vestigial at this point.
A series of three high-sensitivity ground-based receivers—collectively known as NASA’s Deep Space Network—are currently downlinking New Horizons’ data at a plodding 2,000 bits per second...The craft is currently configured in what NASA calls ‘three-axis pointing mode’ (aka, Pluto observing mode), but it’ll transition over to ‘spin-stabilized mode’ after the encounter is over. In spin mode, New Horizons will be pointing itself arrow-straight at the Earth, spinning along its axis for increased stability. As a result, NASA reckons we’ll be able to boost downlink speeds to something in the neighborhood of 4,000 bits per second over the next few days.
"If you’re really geeking out about this stuff, you can go on the Deep Space Network’s website and watch as some of the most sophisticated comm systems on planet Earth literally collect the information, bit by bit,"