The weight of a diamond is measured in carat weight. Technically, all brilliant cut diamonds of the same size should weigh the same carat. This doesn’t translate to other gemstones as the properties are different, so a one carat diamond, will be a different size to a one carat emerald or a one carat sapphire.
The shape of a diamond is really important, as some diamonds shapes create the illusion of looking bigger than they actually are. Marquise, oval and rectangle shaped diamonds tend to look bigger because of their longer shape. The way your ring has been set can also affect how big the diamond looks, halo settings and having a narrow band also help to make the stone appear larger.
This short video explains carat weight and shows how diamonds are weighed in GIA’s lab on an extremely precise electronic scale.