Title says all, basically.
To expound, dozens of empires have come and gone in the past. Some were more vast than others. If one looks at a political map of different dates, it will be seen that empires go from small to large, over the course of years and decades, sometimes even centuries. After all, when Constantinople fell, it was just a vestige of what was once a massive empire that held the entire Mediterranean, and even before that, Rome's territories were lost over the centuries.
This case was not for the Macedonian affair with the Achaemenid Empire. Alexander's empire basically experienced the imperial equivalent of a pubertal growth spurt. After two major engagements with Darius, the entire empire was his.
Note that this is not to take away from Alexander's genius, but I am merely asking whether or not he fought an enemy that was not in top shape.
Yes it was weakened severely. A court eunuch, Bagoas, wanted power, however he wasn't an Achaemenid and so couldn't exactly rule so his solution was to have several of the family members murdered and have one placed in power that he could control. This ended up not working so Bagoas had his appointee killed and had Darius III appointed. So a young inexperienced man has come to power when he was never meant to hold it. In addition Philip II sent Greeks over as an expeditionary force and they wreaked havoc on the Ionian coast. Persia had also just resolved a small civil war. So yes, the empire was in chaos but that may be putting it mildly.
Source: The Greeks by Powell