Forum:Question about the production of the death star

at the end of episode 3, we see palpatine and vader, staring out of an star destroyer at what appears to be the death star in prduction.

this death star took approximatly 19 years to build.

however the death star II seemed to only take 4 years to build, even though it was bigger and more powerful (i know it wasnt completed, but the majority of it was)

does anyone have any theories about this?