The size of an x-ray grows with power for two reasons: higher voltages require much more insulation, and higher energies require much more shielding. In medica

The size of an x-ray grows with power for two reasons: higher voltages require much more insulation, and higher energies require much more shielding. In medical contexts shielding is the big issue. I forget the exact figures but the mass attenuation coefficient for large energy is something like Z/E^3, so a dental x-ray at 15 kev will has much less radiation leakage than a chest x-ray at 150 kev. (Note that the beam is not monochromatic and the 1000x change in attenuation passes through an exponential decay relation so it is not a simple relation, but you need a lot of lead to keep the x-ray tech safe).