Do you instead mean "thick", i.e. in the axial direction of the engine? If so, that depends upon where the alternator's pulley needs to sit. Perhaps it was always on the innermost crankshaft pulley, with 2 belts used for factory AC. I have read that some aftermarket "racing" dampers are thicker so push the crank pulleys out. Anyway, many pulley and belt combinations, so not everyone will have the same issues. I thought the roundback and squareback alternators have almost the same max thickness, though the contours of the bodies vary, so a roundback might squeeze in better some places.
