Why the current in X ray tube is in milli amps
An X-ray tube might require 100kV. A current of only 1A would be 100kW, which is a fairly large amount of power even for a water cooled tube with rotating target.

The efficiency using a tungsten target would be less than 1%, so most of the power goes into heat. 

There's no need to produce more X-rays than required, however CT scans may require something of the order of a 100kW power input. They also can expose the patient to a relatively large amount of radiation. 

Along similar lines, a particle accelerator beam of just a few tens of nA can carry a great deal of power if the particles are energetic enough (say hundreds of MeV).