When you are interested in low power, you must analyze static and dynamic power separately. Static power (when the chip is powered up, but not being clocked) used to be very low, but is significant in the newer smaller-geometry technology. Static = leakage current depends strongly on temperature. Make sure you look at both 25 degree and 85 degree values. Some manufacturers conveniently underreport by mentioning 25 degree values only :-(
Dynamic power comes from the charging and discharging of capacitances, and depends therefore on the clock rate of all the various nodes inside the chip and the I/O. It is proportional to clock frequency, and increases with the square of the supply coltage (the current increases linearily, the power obviously with the square of Vcc). Newer technology reduces internal capacitances, thus use less power at the same functionality and speed. But functionality as well as clock rate often increases, and power might therefore go up anyhow.
Peter Alfke, Xilinx Applications