I think it depends on the particular memory used. It's rather interesting. I suspect that one of the reasons why current 4 GB modules are so expensive is that they use 8 very expensive 4 Gbit (512 MB) parts for each module. Most of the 2 GB modules on the market use 16 common 1 Gbit (256 MB) parts for each module. The 1 Gbit parts are extremely mature and probably are sold at an excellent price point. In fact, I'd say there's a glut of them on the market. The 4 Gbit DDR3 parts have only been on the market for a few months and suffer from "early adopter" prices since they're made with the latest semiconductor processes. As an engineer, I'd note that often the latest processes suffer from yield problems, which get worked out over time.
Here's a Micron 2 GB module datasheet:
http://download.micron.com/pdf/datasheets/modules/ddr3/jsf16c256x64h.pdf
Here's a Micron 4 GB module datasheet:
http://download.micron.com/pdf/datasheets/modules/ddr3/JSS16C512x64H.pdf
In some of the current draw specs, the 4 GB version actually uses less current than the 2 GB version, including refresh current. Even such things as "active standby" actually consumes less power on the 4 GB module compared to the 2 Gb module. It's on page 10 of the 4 GB datasheet and page 11 of the 2 GB datasheet. They operate at the same nominal voltage, so the power draw should be proportional to the current draw.
It's kind of strange since I don't know of any 2 GB parts on the market. I don't think they can simply double up on the parts either (go to 32 memory parts per 4 GB module), since this would add to the loading. What they seem to do is use the latest semiconductor processes, which carry a extreme premium price compared to the more "mature" parts found in 2 GB modules.
