Structural parameters relative to Euclid, one per row: elliptical Petrosian radius R_Pet (top), Sérsic effective radius R_e (middle), and Sérsic index n (bottom). Columns compare E-BGS (left), DESI r band (center), and DESI z band (right) with the corresponding Euclid measurements; points are color-coded by the kernel density estimate (KDE), and the dashed red line is the one-to-one relation. Inset: median bias and normalized median absolute deviation (NMAD) scatter σ_NMAD of the residual Δ ≡ Euclid − method.