Reduction in the spatial extent of the star-forming gas in TNG50 cluster satellites after infall. Left: evolution of the SF-weighted gas radius of the example galaxy of the previous figures, as a function of lookback time in gigayears. The black curve represents the outcome along the main progenitor branch of the galaxy; the blue curve represents a Fourier-smoothed version thereof. The yellow curve highlights the time when the gas radius gets smaller. The solid vertical black line represents the time when the gas radius starts to shrink, which we define as t_trans. The dashed black line represents infall, t_infall. The dashed blue, orange, and red lines represent 1 Gyr, 2 Gyr, and 4 Gyr after t_trans, respectively. Middle: relationship between infall time and the time when the gas radius starts to shrink for all selected TNG50 cluster satellites, colored by the mass of star-forming gas at infall. Gray crosses represent preprocessed galaxies. The solid black line represents the one-to-one line. Right: probability density distribution of the gas radius ratio to that at infall for all selected TNG50 galaxies inspected 1, 2, and 4 Gyr after t_infall, respectively.