Panels (a) and (b): Charged particle $v_2\{2, |\Delta\eta|< 1.0\}$ (left) and $v_2\{4\}$ (right) as a function of centrality in Xe$-$Xe and Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}= 5.44$ TeV and $\sqrt{s_{\rm NN}}= 5.02$ TeV, respectively. Panels (c) and (d): Ratio between Xe$-$Xe and Pb$-$Pb $v_2\{2, |\Delta\eta|< 1.0\}$ (left) and $v_2\{4\}$ (right). Statistical and systematical uncertainties are shown as vertical lines and grey boxes, respectively. The measurements are compared with IP-Glasma+MUSIC+UrQMD calculations  to constrain the $\beta_2$ and $a_0$ parameters of $^{129}$Xe nuclei. The thickness of the bands represent statistical uncertainties.