Jeff Masters at Weather Underground:
Hurricane Ike grew unusually large, eventually filling up the entire Gulf of Mexico and becoming larger than Katrina. How did it get so big? Well, one theory is that the storm’s passage over Cuba helped it to grow in size. During the day and half the eye of Ike traversed Cuba, the thunderstorm activity near the center was suppressed by land. However, a large portion of the storm was over the exceptionally warm waters of the Loop Current on either side of Cuba. Since the storm couldn’t put any energy into intensifying and maintaining its core, the energy pulled out of the Loop Current went into expanding and intensifying the outer portions of the storm that were over water. When Ike finally emerged into the Gulf of Mexico, its scale had been reset to this new larger size, and the storm was able to maintain the new scale. A similar transition to a new larger scale also occurred to Hurricanes Katrina and Andrew after they passed over South Florida.