At the risk of repeating the same mistake as my previous comment, I’ll do armchair genetics this time:
Perhaps genes controlling the size of various mammalian organs and body regions tend to grow or shrink uniformly, and only become disproportionate when there is a stronger evolutionary pressure. When there is a mutation leading to more growth, all the organs tend to grow more.
At the risk of repeating the same mistake as my previous comment, I’ll do armchair genetics this time:
Perhaps genes controlling the size of various mammalian organs and body regions tend to grow or shrink uniformly, and only become disproportionate when there is a stronger evolutionary pressure. When there is a mutation leading to more growth, all the organs tend to grow more.