Among other things, they do less per clock, which likely means fewer adders and other compute units, smaller storage to support a smaller amount of out of order operations, etc.

The E-Cores are an extension of the Atom cores, so you may be able to find older articles comparing their die size and structure to 'Core' cores. They've been about 1/3rd to 1/4th the size and around 1/2 the performance of similar age designs through most of their lifetime.