[BMCDA]

Because there's another head before the baby's that prefers the canal to be smaller ... ?

You may have a point there ;-D

[Gumlegs]

Thanks. I think that at the very least you have to admit that it's a plausible hypothesis.

[DennisR]

It's a miracle in itself that BOTH heads, so disparate in size, use the same canal for completely different means. Pretty amazing, if you ask me. And also excellent design: two for the price of one!