babyjellyfish if they're same sex and same gestation they'd usually be similar sizes with a slight trend statistically towards subsequent babies being slightly heavier.
Boys are usually heavier than girls at birth if all other considerations are the same.
I had one 42 weeker and 2 38 weekers and although the two born at 38 weeks are 3 years apart they were the same weight almost to the gram/ oz! Obviously the 42 weeker was heavier. As teens their sizes don't correlate to their comparative birth weights but their lengths at birth do (typically babies are measured as well in many countries outside UK at birth).