You're correct in that IVs make a bigger difference on smaller stats, since it's the relative increase that the IV gives which matters.
On Chansey the biggest boost is going to be having 15 attack IV, since it's a 1.25x increase to it's base attack and therefore a 1.25x increase in max CP compared to a 0 attack IV Chansey.
15 stamina is only a 1.03x increase to base stamina and therefore only a (1.03^0.5) = 1.01488916x (or 1.5%) increase in max CP, so even though your Chansey only has 26/45 IVs, it's going to have about 97.5% max CP compared to a perfect one, because it's got the best possible IV distribution despite it's total IVs being pretty average.