We measure the weight of diamonds in carats because it is the most universal measurement in such small units. Don’t confuse Carats with Karats- gold purity.
Carats tells us the weight of a diamond, but not necessarily the size of the stone. Depending on how a Diamond is Cut, determines how big it actually looks. For example in the GIA reports below, a 0.90 carat stone could have a bigger size dimension than a poorly cut 1.00 carat stone.
Excellent Cut 0.90 carat = 6.31mm compared to
Very Good Cut 1.00 carat = only 6.14mm