UK currency laws specify the weight of coins, and banks here weigh coin to check on the value of a bag that is being paid in, rather than using rolls as in the USA. As a result, if a new metal is used then the volume will differ; if the area of the coin is the same then the thickness will increase in the inverse ratio of the densities.

Ignoring the coating, iron (& mild steel) has a density of 7.87 g/cm^{3}. Cupronickel is around 8.9 g/cm^{3}. The ratio of these two is 1.13; in other words the coins will be 13% thicker, a noticeable difference. The ratio is more or less the same for the 'bronze' coins.