The spot radius being displayed here is based only on the geometric rays and doesn't account for the diffraction limited behavior of your system. Since your spot is smaller than the Airy disk, that's one indication that your system is diffraction limited and you will likely instead want to use other analyses such as the PSF to get a more realistic understanding of your system performance.

Thank you very much for your answer!

No problem! FYI, the reason that we have both a geometric spot radius analysis as well as other analyses that consider diffraction (like PSF) is because the geometric calculation is MUCH faster and it's accurate for cases with sufficient aberration. But, as you've illustrated, there are also times when the geometric analysis isn't an accurate reflection of the system because it predicts overly optimistic performance. So if you're not sure whether your system needs to account for diffraction behavior, a general recommendation is to compare the results of the two similar analyses (Spot v. PSF, for example) and if they differ significantly, go forward with the diffraction calculation. We have more examples and guidance on this in the Optical System Design Using OpticStudio course available in OpticsAcademy.