The E-Pace should really be compared to the UX with both being around 4.4m long.
The NX is closer to the F-Pace both being roughly 4.7m long
So it makes sense that the NX would be more expensive as a used buy, more so as its quite popular in the range.
Additionally, you seem very hung up on the mpg? Even if you managed 56mpg all the time in a UX and 41mpg in an NX, it'd only save you £400 over an entire year. Dont buy a vehicle based on the mpg as its such a small difference at low miles anyway, thatd be my advice