Dan Balz explains,
Here is a simple way to understand the consequences of that choice. Take two states that held Super Tuesday contests on Feb. 5: big New Jersey, with 107 pledged delegates at stake, and tiny Idaho, with 18 delegates up for grabs. Clinton won New Jersey's primary and made headlines for doing so early on that night, while Obama won Idaho's caucuses long after many of those watching had gone to bed. But because of the rules of proportionality, Clinton netted just 11 more delegates than Obama from her New Jersey victory, while he gained 12 more than her by winning Idaho.
That pattern held through other states on Feb. 5 and Feb. 9, as Obama rolled up substantial margins and, as a result, harvested delegates in numbers that belied the relatively small size of some of the states. Eight states held caucuses during that period -- Alaska, Colorado, Idaho, Kansas, Minnesota, Nebraska, North Dakota and Washington -- and together awarded 305 pledged delegates. By the Obama team's calculations, the split out of those states is about 209 for him and 96 for Clinton -- an advantage of 113 delegates.