Why not always take a very small focal spot?
The physics of a X-ray tube set the limits.
Only 2% of the energy is transformed to X-ray quants, about 98% is heat, which will be on the small focal spot. The target (anode) where the electron beam is focused to is mainly made of tungsten due to the high melting point (3,422 °C). Tungsten is not a good heat transmitter, therefore it is embetted in copper, which could transfer the heat much better. Here is apicture of a tungsten target in cooper, destroyed by heat:
The copper itself is cooled with flowing water to bring the energy to somewhere else.
The energy density of the focal spot is about 1000W per square mm with a target of 20°. If the target is steeper, the size of the tungsten is larger; at 11° about 1800W can be used with such a tube. (hint: with 1mm*1mm focal spot size a 45° target would have the dimension of 1mm*1,17mm, a 20° target already has 1mm*3mm and a 11° target 1mm*5.7mm and the larger size could transfer more heat).
More power produces more dose. A higher dose rate lead faster to the image or give less noise in the image. The detail visibility also depends on noise, as shown in the thread
.
My favorit image to show the difference is the BAM5 weld:
Finally it is a trade off in the triangle between high dose or high spatial resolution or fast image capture - which is the content of the thread
.