Metal-halide perovskite semiconductors have attracted intense interest over the past decade, particularly for applications in photovoltaics. Low-energy optical phonons combined with significant crystal anharmonicity play an important role in charge-carrier cooling and scattering in these materials, strongly affecting their optoelectronic properties. We have observed optical phonons associated with Pb-I stretching in both MAPbI(3) single crystals and polycrystalline thin films as a function of temperature by measuring their terahertz conductivity spectra with and without photoexcitation. An anomalous bond hardening was observed under above-bandgap illumination for both single-crystal and polycrystalline MAPbI(3). First-principles calculations reproduced this photo-induced bond hardening and identified a related lattice contraction (photostriction), with the mechanism revealed as Pauli blocking. For single-crystal MAPbI(3), phonon lifetimes were significantly longer and phonon frequencies shifted less with temperature, compared with polycrystalline MAPbI(3). We attribute these differences to increased crystalline disorder, associated with grain boundaries and strain in the polycrystalline MAPbI(3). Thus we provide fundamental insight into the photoexcitation and electron-phonon coupling in MAPbI(3).