Miarex, there is no one specific answer to this question. There are many different reasons why twins can be different sizes. Although you may have observed the described scenario (firstborn twin smaller than second born) to be commonplace, I don't believe that, statisically, it would prove to be the standard. (I'm not aware of any recorded research on this issue; it would be interesting to see some real numbers!) In many cases, the second born twin is smaller, while in other cases, one twin is smaller at birth but outpaces their twin as they grow up. Birth order doesn't really impact the size of the babies.
However, to address your question about why any two twins would show a disparity in weight and size, there are several reasons. For fraternal (dizygotic) twins, the answer is simple genetics. Just as any two siblings may be taller or shorter or thinner or wider, fraternal twins inherit their own unique set of DNA characteristics that influence their size. Second, environmental influences may be at play. Monochorionic twins who share a placenta may be in competition for nutritional resources, and the twin at a disadvantage experiences some level of growth restriction. Twins with TTTS (Twin-to-Twin Transfusion Syndrome) will often be of different sizes. The bottom line is that for every set of twins who are different sizes, there are as many reasons to explain the disparity.
Miarex, Thanks for posting your question. I'm sure it will spur some interesting discussion! Please stop by the message forum and share your thoughts. Are your twins different sizes? Is the firstborn twin the larger one or the smaller one? Why do you think this is the case?
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• Why Are Identical Twins Different?
• Birth Order and Twins: Does it Matter?