Lab diamonds have the exact same mineral as natural Diamonds. However, it is normal for a lab-created diamond to vary slightly in weight from the "ideal" weight. For instance, a 1ct ideal round brilliant cut diamond will weigh within 0.02ct of 1ct (.18-.22). However, many labs can create diamonds that are very close approximations of natural diamonds in terms of weight. For this reason, you can expect lab-created diamonds to be near the same size as natural diamonds, if not necessarily the exact same size.