OK but that comparison doesn't apply to me. I use Fujifilm cameras and lenses. [...]
Fujifilm X is indeed a great young system, BUT:
1. I cant handle these tiny cameras. Too small for my above average large hands to hold comfortably, let alone operate them comfortably.
2. I can get as good or better glas cheaper for Nikon. Granted you have to find out which they are (*)
3. I like the advantages of Nikon - like excellent high ISO abilities, or their flash system.
Also yes, for getting all out of Nikon, you need full frame. Otherwise all these great lenses you can get will be cropped.
(*): Hint: Usually (sadly) not the newest stuff. Those are mostly to keep the audience entertained. Try for example those lenses they produce since 20, 30+ years ... for a reason. Theres of course also pearls they didnt produce for too long, though. The majority of the lenses I use daily are Zeiss/Voigtländer or decades old Nikkor.
If you want a camera that big and heavy you might as well use a DSLR.
See ? Sensor size determines size of camera + lens - well at least the length, a DSLR has a pentaprism above and an AF sensor assembly below the mirrorbox, so yeah they are a bit higher and heavier.
The other reason however why Fujifilm is so much more lightweight than APS-C DLSRs is simply that all APS-C DSLRs are crop cameras - their mirror box has to be larger because the original format of that system is full frame aka small format.
Fujifilm X however is native APS-C. That means 24x16mm is NOT a crop sensor when using Fujifilm X, but its the full frame of that system. That means everything is only as large as it has to be, and not one bit larger.