Each stat has a "base stat" and then an Individual Value (IV); Each IV for attack / defense / stamina is added on to the pokemon's base stats for each, giving you a final value, so to speak.
Dragonite, for example has base stats of 263 (atk), 201 (def), and 182 (stam), so if the pokemon's IVs were 15-15-15, the final values would be 278, 216, and 197.
Thus, the stamina IV gives the largest proportional gain to that respective stat for Dragonite. A 15 IV in stamina will give a larger % increase to that stat than a 15 in the Attack IV would give to Atk. The Attack IV; however gives the biggest increase to maximum CP.