Carat means the weight of the diamond, not the millimetre size. It’s important not to focus solely on the carat weight though, because taken as an individual factor, carat weight doesn’t determine a diamond’s value. If you took two one-carat stones and compared them, they could vary widely in price when clarity, colour and cut were taken into consideration. A diamond that is a very deep stone may have a higher carat weight but not actually appear any larger from the top view as a smaller carat weight stone, being very deep this will also affect the quality of the cut and the stone may not be as sparkly as a diamond with ‘excellent’ proportions.
Carat weight can appear differently on different shape stones, so if you are looking for a larger size without compromising on quality, I can suggest shapes that will be perfect for you.