First of all, bigger babies are not being caused by better nutrition. Maternal diabetes is by far the most common cause of LGA babies. Diabetes during pregnancy causes the mother's increased blood glucose to circulate to the baby; the baby then makes insulin. All the extra sugar and the extra insulin production can lead to excessive growth and deposits of fat, thus, a larger baby.
Solution Pat A, prevent or treat maternal diabetes.
Second, twinning the conceived child wouldn't help. Where did you get that idea? Monozygotic twins develop within 2 weeks of fertilization, i.e. almost always the twinning split occurs before the mother knows she is pregnant.
If she's diabetic and carrying twins, she could have TWO large-for-gestational-age babies in there at once.
And five? That's almost certainly the result of either hyperovulatory drugs, or IVF.
Solution Part B, ban hyperovulatory drugs, ban IVF.
I’ll readily cede to the better informed.