As you say, not so simple.......but can be seen that way!
Your 5D2 full frame has 21MP sensor but for example the 80D crop sensor is 24MP, you have more MP on a smaller sensor.....
compared to say cropping in on a 5D2 image you might have 15MP image.
In real terms it matters little but there are downsides to consider ~ the 5D2 will(?) give you cleaner files @ higher ISO's compared to the same ISO on the 80D.
The cropped 5D2 file because it is lower noise will be a good candidate for increasing its size!
Ultimately, what is your intended use ~ web only or larger prints.
Just some things to think about above the simplistic 'view'