To generate more X-ray quants the focal spot is positioned in a large angle towards the filament; the effective focal spot (seen if ) is much smaller as the thermal focal spot where the electrons are absorbed.

A small target angle (seen perpendicular onto the focal spot) allows a larger area for the same size of effective focal spot but the effect is more relevant.

The focal spot size depends on the angle in the beam - in both directions.
The nominal focal spot size is only valid for 0° / 0° (in the center of the beam). An uniform focal spot size not possible in the full beam, in the result the geometrical unsharpness due to focal spot depends on position of object in the beam!

To show the effective focal spot shape in the full beam a 1mm thick Tungsten plate was created with 25 holes

and the central nine holes were imaged with a magnification of eight with the large focal spot of a 225kV tube with a target angle of 11°. The central hole shows a focal spot size of about 400µm (*200µm) in all four directions (measured with the ):

but in the "corners" the focal spot shape look different

and with the evaluation results of the corner focal spots

the different sizes in the beam is showed: The size in direction of the electron beam increases from 262µm (*131µm) at the right side to 592µm (*296µm) on the left side.
* due to a mistake in scaling a factor of two has to be applied to the printed values.

As the effect is stronger as smaller the focal spot would be, a calculation for a tiny focal spot of nominal 0.2mm [ ] in both directions in the central beam was done. The target angle was 11°, the angle for the test was +/-20° (N->S) +20°/-10° (W->E) and covers the full active area of beam.

A tube similar to the calculation was built and the focal spot size was tested using the ASTM E1165 user method for the calculation of the tiny focal spot. In difference to the post before now the minimum and maximum angles are shown.

The measured values differ from the calculated values. Even if the real values are not as bad as calculated there is a factor of 2 still. Hint: The values are smaller compared to the EN12543-2 - the reasons are explained .
When you measure the focal spot size you should have in your mind that the given focal spot value is for the central beam only.
The good news: A larger angle of the target would reduce the effect and the trend for smaller targets is going back to the target angles of about 20° .