Oil field size is not the only factor, the most important is how easily the oil in the field can be collected. Some rock is much more porous than other and thus allow the oil in a field to flow more easily to the collection point. If the rock is less porous, you have to drill more wells in order to extract it. More wells = more costs. Doubling production probably involved drilling more production wells.
Anything off-shore is expensive, to have to drill lots of wells offshore is even more expensive. Drilling lots of wells on land is cheap(er). Problem is, the large majority of significant US oil and gas deposits are offshore or in Alaska. Both are fairly expensive areas.
Winter drilling season is upon us soon, I know the rigs in Canada are going to be working full tilt...
Soda