Do twins start out smaller or just at the end?

I know twins are usually alot smaller than an average baby at birth because there are two of them and they needed to be smaller to both fit. But I'm wondering if twins start out small in the beginning and stay smaller than an average baby throughout every stage in pregnancy? Im curious cause i'm wondering if with twins you might not be able to hear the heart beats as early as with most babies because they are smaller and need to wait and extra week or so to be at the size where u can hear the heart beat. Or should u be able to hear twins just as early as a single baby?