To determine the spread or how large a diamond appears, I always recommend looking at the measurements of the diamond rather than its carat weight. The millimeter measurement refers to the actual physical dimensions of the diamond. It takes into account the length, width and depth of the diamond, which are all important factors that affect its appearance. A diamond with larger measurements will generally appear larger and more impressive, even if it has a lower carat weight.
However it is super important to consider the proportions, including the depth, table size, crown angle, and pavilion angle, as these factors greatly affect how well the diamond reflects light and sparkles, which is what makes a diamond truly beautiful and valuable. Depth is very important, a diamond cut too deep or too shallow can cause light leakage which can can determine the sparkle or dullness of a diamond. In these cases, light travels through the diamond and escapes out the sides, instead of reflecting back through the table to your eyes.
The Bottom Line
Bottom line is, the 4 C’S are just a starting point. The measurements are a more accurate indicator of a diamond's physical size and overall appearance, talk to an expert to make sure you are spending your money where it counts and are getting a diamond with fire and brilliance.