One of the Macedonian military innovations was making the successful trade-off between the personal safety of the individual Hoplite and the tactical safety of the entire phalanx - at least in the face of missile troops.

Macedonian Hoplites were more lightly armoured than their Greek counterparts -making them more vulnerable to missile fire - but this was offset by the presence of numerous cavalry units and aggressive cavalry tactics. At Granicus, Macedon fielded over 5,000 cavalry, including the elite Companions, Thessalians, Allied Greeks and Paeonians. The presence of so many cavalrymen limited the use of enemy missile troops, who were particularly vulnerable to cavalry charges and tended to rout when pressed. This reduced the amount of time that the Macedonian phalanxes were subject to missile fire.

Moreover, the Macedonians favoured shock tactics and used cavalry to both smash the enemy centre (as at Granicus and Issus) or to flank enemy lines and draw troops from the centre (as at Gaugamela). Alexander the Great practiced an aggressive form of hoplite warfare, where infantry would advance rapidly and engage the enemy, while cavalry would draw off units to the flanks (thus weakening the enemy line) or charge the centre, forcing a rout. The infantry would act as a holding force, keeping the enemy formations in place while the cavalry saw off their opposite numbers and organized a devastating charge. In either case, Macedonian infantry units would spend less time under missile fire than traditional Hoplites.

It's essentially a difference in tactics and army composition. A Greek Hoplite fought at an infantryman's' pace, with the attendant risk of sustained missile fire - hence the heavier armour and larger shield. The phalanx was expected to carry the day and cavalry was mainly used for harassment or running down fleeing enemies. A Macedonian fought as a component of a mixed force, fusing rapid cavalry action with the slower advance of the phalanx. Instead of relying on armour or shields as protection, the Macedonians fought a different kind of Hoplite warfare - one where speed and aggressiveness reduced their vulnerability to missile troops.

When you think about it, it would seem that the phalangites were significantly more vulnerable to missile weapons than the traditional hoplite. Lighter/less armor, consisting of a leather cuirass and a helmet, and a small, dinner plate sized shield strapped to the upper arm. Doesn't seem like very much protection. Also, I don't think spears would provide much of a screen, as they are rather narrow, it would be difficult to block something like an arrow.