It was recently discovered that molecular ionization at high x-ray intensity is enhanced, in comparison with that of isolated atoms, through a phenomenon called CREXIM (charge-rearrangementenhanced x-ray ionization of molecules). X-ray absorption selectively ionizes heavy atoms within molecules, triggering electron transfer from neighboring atoms to the heavy atom sites and enabling further ionization there. The present theoretical study demonstrates that the CREXIM effect increases with the size of the molecule, as a consequence of increased intramolecular electron transfer from the larger molecular constituents attached to the heavy atoms. We compare x-ray multiphoton ionization dynamics of xenon, iodomethane, and iodobenzene after interacting with an intense x-ray pulse. Although their photoionization cross sections are similar, iodomethane and iodobenzene molecules are more ionized than xenon atoms. Moreover, we predict that the average total charge of iodobenzene is much larger than that of iodomethane, because of the large number of electrons in the benzene ring. The positive charges transferred from the iodine site to the benzene ring are redistributed such that the higher carbon charges are formed at the far end from the iodine site. Our first-principles calculations provide fundamental insights into the interaction of molecules with x-ray free-electron laser (XFEL) pulses. These insights need to be taken into account for interpreting and designing future XFEL experiments.X-ray free-electron lasers (XFELs) have opened new horizons for studying the structure and dynamics of matter with femtosecond temporal and atomic spatial resolution [1][2][3]. With the rapid advance in XFEL technology, the peak intensity already exceeds 10 20 W/cm 2 in the hard x-ray regime [4]. At extremely high x-ray intensity, it is crucial to understand the nonlinear interaction of atoms [5][6][7][8][9], molecules [10][11][12][13][14][15][16][17], and clusters [18-20] with xray fields. The interaction between matter and intense x-ray pulses can be described by the multiphoton multiple ionization dynamics model. An x-ray photon ionizes an atomic inner-shell electron, followed by relaxation processes. During the intense x-ray pulse the photoionization and accompanying processes repeat sequentially. This ionization model has been verified for atomic systems with a series of gas-phase XFEL experiments [5,7,8].When a molecule is irradiated by x rays, they mainly interact with heavy atoms within the molecule, because their photoionization cross section is much higher than that of light atoms. It had been widely believed that ionization dynamics of a molecule exposed to XFEL irradiation is similar to that of an isolated atom whose cross section is comparable to the molecule [12][13][14][15]. This expectation turned out to be valid in the low-fluence regime. For example, the total molecular ionization of methylselenol (CH 3 SeH) at low x-ray intensity, where single-photon absorption is dominant, was found to be similar to the atomic ionizat...