The greater the number of urban customers, the larger the base to spread the largely fixed infrastructure and overhead costs over and the lower the per customer charge should be.  If you look at the density of the Eugene population versus the density of the Springfield population,  EWEB has a large advantage over SUB.  Where EWEB loses in the infrastructure comparison is the minority of customers it serves up river.  SUB service boundaries  pretty much stop at the city limits, while EWEB has electric customers up past Vida.  They have tried to trade off that service area several times but no one wants it.
Overhead is overhead.  There is no comparison between the overhead (look at the the SUB headquarters facility and the EWEB headquarters).  When EWEB first built that palace, they even hired a chef for the employee cafeteria!