We theoretically studied the electronic and electrical properties of metallic and semiconducting peapods with encapsulated C_{60} (C_{60}@CNT) as a function of the carbon nanotube (CNT) diameter. For exothermic peapods (CNT diameter > 11.8 A), only minor changes, ascribed to a small structural deformation of the nanotube walls, were observed. These include a small electron charge transfer (less than 0.10 electron) from the CNT to the C_{60} molecules and a poor mixing of the C_{60} orbitals with those of the CNT. Decreasing the diameter of the nanotube leads to a modest increase of the charge density located between the C_{60}s. More significant changes are obtained for endothermic peapods (CNT diameter < 11.8 A). We observe a large electron charge transfer from C_{60} to the tube, and a drastic change in electron transport characteristics and electronic structure. These results are discussed in terms of pi-pi interaction and C_{60} symmetry breaking.
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