The Makedonian phalanx -- why such depth?

[Paralus]

Another passage for thought.

Livy 37.40(Battle of Magnesia)
There were 16,000 infantry in the Macedonian fashion known as the "phalanx." These formed the centre, and their front consisted of ten divisions; between each division stood two elephants. They were thirty-two ranks deep. This was the main strength of the king's army and it presented a most formidable appearance, especially with the elephants towering high above the men.

I would presume that this was the depth in which they fought given Livy's description - unless he is presuming that his readers knew that the phalanx would "halve its depth" to 16 when closing up for action?

Another question. Just how deep was the “mixed phalanx” of 16 per file going to fight? Arrian mentions no 'half file' leader. If a 'half file' leader did exist - as might be demonstrated by Alexander stretching his line at Issos - that would only serve to indicate that the normal file depth for fighting was 16.