The Makedonian phalanx -- why such depth?

[Paullus Scipio]

As I said to begin with, my reasons for thinking that Greek authors when describing depth speak of open/normal order, (4 cubits/6 feet per man), which depth is subsequently halved just before combat when they 'close up' into pyknosis/close order( 2 cubits/3 feet per man), are quite complex - far too many to set out succinctly here.( it would require an in-depth analysis of various sometimes lengthy passages, cross-referenced to the manuals)

The passage of Polybius (XII.21) you refer to is one of the clues. Suffice to say it is a common fallacy that Polybius/Callisthenes has Alexander 'broaden' his phalanx to fill the width of the battlefield. In fact, Polybius scorns Callisthenes because even in close order 8 deep, the phalanx would be too wide to fit into the narrower battlefield (on Callisthenes stated distances). Bear in mind that a phalanx 16 deep, on closing to 8 deep will be exactly the same frontage/width as before.....