Panels (a) and (b): Charged particle $\langle v_2 \rangle$ (left) and $\sigma_{v_2}$ (right) as a function of centrality in Xe--Xe and Pb--Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 5.44$ TeV and $\sqrt{s_{_{\mathrm{NN}}}} = 5.02$ TeV, respectively. Panels (c) and (d): Ratio between Xe--Xe and Pb--Pb $\langle v_2 \rangle$ (left) and $\sigma_{v_2}$ (right). Statistical and systematical uncertainties are shown as vertical lines and gray 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.