Toy model of how (upper left), (upper right), r_{1/2, 3D, baryons}/R_e, disk (lower left), and (lower right) vary with redshift for a range of fixed log₁₀(M_*/M_⊙), using “typical” galaxy sizes, intrinsic axis ratios, gas fractions, B/T ratios, halo masses, and halo concentrations (from empirical scaling relations or other estimates; Dutton & Macciò 2014; Lang et al. 2014; van der Wel et al. 2014; Moster et al. 2018; Übler et al. 2019; Tacconi et al. 2020; Genzel et al. 2020). The assumed (interpolated, extrapolated) property profiles as a function of redshift for each of the fixed log₁₀(M_*/M_⊙) are shown in the top panels. Using abundance-matching models (inferred from Fig. 4 of Papovich et al. 2015, based on the models of Moster et al. 2013), we show the path of a Milky Way (MW, M_* = 5 × 10¹⁰ M_⊙ at z = 0; black stars) and M31 progenitor (M_* = 10¹¹ M_⊙ at z = 0; grey squares) over time in each of the panels, assuming the progenitors are “typical” at all times. This inferred “typical” evolution would predict an increase in f_DM(R_e, disk) with time at fixed M_*, with lower masses having higher f_DM at all z. The evolution of the structure and relative masses of the disk, bulge, and halo predict an increase (M_* ≳ 10^10.25 M_⊙) or “dip” (M_* ≲ 10^10.25 M_⊙) in the difference between z ∼ 0 and z ∼ 0.75, and then an increase until z ∼ 2 when the difference flattens (largely reflecting the flat q_0, disk estimate for z ≳ 2). The difference is minor, between ∼ − 0.025 and −0.005 for the stellar masses shown. The ratio of the composite r_{1/2, 3D, baryons}/R_e, disk increases with redshift for all masses, with more massive models predicting smaller ratios at each z. The MW and M31 progenitors have f_DM(R_e, disk) evolving from ∼0.33 and ∼0.25 (respectively) at z = 3, decreasing to ∼0.25 and ∼0.2 at z ∼ 1.5, and then increasing to roughly same value ∼0.45 at z = 0. The and values are relatively similar down to z ∼ 1.5, but at lower redshifts (where r_{1/2, 3D, baryons}/R_e, disk ≲ 0.9) the difference increases up to ∼0.065 (MW) and ∼0.14 (M31) at z ∼ 0. While this “typical” case predicts f_DM offsets of only 0.035 at z = 2 and increasing to 0.14 at z ∼ 0 for the most massive case, objects with even larger bulges (B/T > 0.4) or radii above the mass-size relation will have even more discrepant f_DM values when adopting these radii definitions (see Fig. 7).