If the moon had formed at the same time as Earth, then the size of the moon's core should have been proportionally about the same as Earth's core.
(the moon; had formed; T:at the same time as Earth) (the size of the moon's core; should have been; T:proportionally; about the same as Earth's core; T:then)
the moon the size of the moon's core
had formed should have been
T:at the same time as Earth
T:proportionally about the same as Earth's core T:then