How big does ground need to be? Step down from 5v10a to 1.2V 50a, do I need a 50 amp ground?
Desktop cpus can take 90 watts, some 120 watts. That's close to 100
  amps at 1 volt. How can they return 100 amps to ground? Does the
  voltage regulator up convert ground somehow so that 100 amps doesn't
  go back through the mains circuit?


In a circuit the current must return back to the source, so the current returns back to the power supply of the computer. This means that the return current pathway must be low resistance with multiple grounds on the ATX cable and a large grounding plane with copper. The current back to the computer power supply is the amount that the DC to DC converter uses, and there are several voltages to choose from like 12V, 5V and 3.3V that can be stepped down by a DC to DC converter. There are also multiple converters. 

The return current to the supply is also calculated at the DC/DC converter, so if you have a 12V DC/DC converter that steps down to 1V and your using 90W only 7.5A would be returned to the ATX supply on the ground, not 90A. For the cables, you would need to keep them low impedance but not that much. Most power supplies use 22ga to 16ga wire, this keeps the resistance in the 10mΩ's range. If you only had one 18ga wire that was 2ft long, that would be 12mΩ. 100Amps would be P=I^2*R so 7.5^2*0.012=0.675W, however, there are more cables, some ATX supplies have more than 10 grounds so the resistance would be 10 times lower, 7.5^2*0.0012=0.0675W dissipated in the cables.


  What if you tied the ground of a voltage regulator to the same ground
  as a 1 volt 100 amp circuit? So you have 5 volts 10 amps and 1 volts
  100 amps returning via the same ground. How does ground not blow up?


The ground does not 'blow up' because the resistance is kept low, there are multiple converters. On a typical ATX board, you will notice multiple DC DC converters. Even with a large return current from the processor, with multiple converters, the return current will be paralleled between the converters. In the simplistic case of your 100A 1V example, with 16 converters each would only get 6.25A.  


https://en.wikichip.org/wiki/voltage_regulator_module

If you do need to run amps of current, the cross sectional area of the conductor needs to be increased to reduce heating. A copper trace with 0.5oz copper and 100mil wide will only heat to 40C above ambient with almost 6A of current through it. Ground planes are much larger/wider than that.