Elongation (aka strain) is a function of the stress in a spoke. The stress is related to two things: tension and cross-sectional area. Tension is a force, cross-sectional area is obviously related to the diameter. When you tighten the spoke, you force it to elongate by the amount you've pulled through the spoke nipple, and the tension increases in proportion with that. For a given spoke, more elongation will mean more tension, but for several different spokes of various different cross-sectional areas, the same elongation (i.e. the same number of turns of a spoke nipple) will produce lower tensions for spokes with less cross-sectional area.