An i7 actaully has 4 cores, but each core has two hyperthreads (which means it can switch between two tasks really quickly). To the OS it looks like 8 cores, so the distinction is probably not relevant here. Sorry. :-)
OK, so the operating system can use the i7 to effectively do 8 things at once. But that assumes that it has 8 things at the one time to do. If it only has one thing to do, then it will only use 1 core.
This is where threading comes in to play. Programs can be written to use threads - GIMP and ImageMagick are two programs I know that do this. When they have a complicated task to do, they split to work up and execute it in different threads, which the OS assigns to different cores. Try setting ImageMagick to work on processing a really large JPEG, and you will see all 8 cores get busy.
Now, I am not sure if gedit is multithreaded or not - but even if it is, the main work when opening a 70M file is processing the disk interrupts as the file is loaded, and I think that work gets restricted to one core.
What can you do about it? Just get busy, multi task, have your computer do lots of things at once, then the load will be spread out over the cores.