Mean lobe surface brightness estimates B_ν at ν_ref = 150 MHz for LoTSS DR1 RGs at z < 0.2, shown as a function of projected proper length l_p. During the majority (∼90%) of their lifetime, RGs grow while maintaining a roughly constant radio luminosity. If, additionally, growth is shape-preserving, surface brightness decreases quadratically with l_p. The seven golden lines therefore denote approximate evolutionary tracks of RGs at z = 0 with end-of-life radio luminosities l_ν ∈ {10²¹ W Hz⁻¹, …, 10²⁷ W Hz⁻¹}. The region of parameter space where RGs can become detectable giants, l_ν ≥ l_ν, min ≔ 10²⁴ W Hz⁻¹, is shaded gold. The region of parameter space occupied by giants, l_p ≥ l_p, GRG ≔ 0.7 Mpc, is shaded green. The region of parameter space inaccessible to the LoTSS, B_ν < σ_Iν = 25 Jy deg⁻², is shaded red.