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Laser-driven proton acceleration from ultrathin foils is investigated experimentally using f/3 and f/1 focusing. Higher energies achieved with f/3 are shown via simulations to result from self-focusing of the laser light in expanding foils that become relativistically transparent, enhancing the intensity. The increase in proton energy is maximized for an optimum initial target thickness, and thus expansion profile, with no enhancement occurring for targets that remain opaque, or with f/1 focusing to close to the laser wavelength. The effect is shown to depend on the drive laser pulse duration.