Top panel: evolution of blob size (R), versus distance travelled by the blob across the jet (Δ_r). The grey shaded area represents the evolution of R for a blob expanding with constant velocity, with a range of β_exp = [0.001 − 0.3], for a Bulk Lorentz factor of 25, an observing angle of 1.5 deg, t_exp = 0, R₀ = 5 × 10¹⁵ cm, and R_H0 = 1 × 10¹⁷ cm. The green shaded area represents the corresponding Δ_r interval for a range of observed delay of days. The blue shaded area represents the same trend for the case of R_H0 = 1 × 10¹⁸ cm. R_H0 is the site of the γ-ray flare. The blue dashed line represents a conical jet profile for R_H0 = 1 × 10¹⁷ cm and the orange dashed line for R_H0 = 1 × 10¹⁸ cm, both starting from an initial value of R₀ = 5 × 10¹⁵ cm. The yellow, blue, and purple shaded regions represent the for Mrk 421, Mrk 501, and 3C 273, respectively. Bottom panel: same as in the top panel, replacing the spatial units with the Schwarzschild radius for a BH mass of 10⁹ M_⊙.