As Nick said, the upper limits are primarily based on the motor, especially with earlier models. I've pushed my older CPP2 with a 3000 series drum and a lot of chemistry, and you can definitely hear the motor struggle with the load. I try not to do that too often.
Check film/developer manufacturer sites for minimum chemistry requirements for various films. The number Jobo cites are minimums required to guarantee fluid coverage of the film in the tank, not developer capacity. You may find in many cases that you need more than the minimum indicated by Jobo. You may even find, for example, that a fully loaded tank can't hold sufficient chemistry to process the film (I haven't run into this yet, but it is possible with certain film/chemistry/dilution combinations I'm sure). Check carefully - Jobo processors aren't failsafe machines.