Bulk density vs. size (volume equivalent radius) of the primary in the ten most massive binaries in the Kuiper belt. The binaries are labelled using the primary-secondary initials: P and C, Pluto and Charon (Nimmo et al. 2017); ED, Eris-Dysnomia (Sicardy et al. 2011; Holler et al. 2021b); HH, Haumea-Hi’iaka (Ortiz et al. 2017; Dunham et al. 2019); MM, Makemake-MK2 (Ortiz et al. 2012; Brown 2013; Parker et al. 2018); GX, Gonggong-Xiangliu (Kiss et al. 2019); OV, Orcus-Vanth (Brown & Butler 2018); SA, Salacia-Actea (Brown & Butler 2017); VI, Varda-Ilmarë (Grundy et al. 2015; Souami et al. 2020); GG, G!kúny||’hòmdímà-G!ò’é!Hú (Grundy et al. 2019). The grey boxes represent the uncertainties. The different size and density estimates for Quaoar are colour-coded, as follows: cyan: Brown & Butler (2017); light blue: Fornasier et al. (2013); dark blue: Braga-Ribas et al. (2013); purple: Pereira et al. (2023); orange: this work, TPM model OB1; pink: this work, TPM model OB2; red: this work, TPM model TC (triaxial ellipsoid with D_eq = 1090 km). The original density estimate of ρ = 4.2 g cm⁻³ by Fraser & Brown (2010) is far outside the borders of this figure.