This was a worry in regards to the republic. It's why armies were not allowed inside the city walls; Pompey, Crassus, Julius et al were supposed to enter the city without their armies.

The difference for Octavian was that at the time he seized power the republic had fallen into disarray -- the Tribunes were now typically patricians, so the counterbalancing power of the plebs was pretty effectively eroded. In addition the years of accumulation of war booty had removed the citizens from the need to support the power structure -- most of the work was done by foreigners and slaves.

In addition the structure of imperial (that is generals') power throughout the empire created large power structures outside the republic's structure. So a large army like Pompey's or Julius's outside the walls (as described above) could become, instead of a guarantee of neutrality, a threat.

Remember in those days one became wealthy through power as opposed to powerful through wealth. Julius became wealthy by being appointed governor of Hispania (IIRC) then then using that as a power base to accumulate more power and treasure. This imbalance I talked about became so large that when Octavian took power (having inherited both wealth and position from his fellow consul and adoptive father, Julius Caesar) he was wealthy enough to pay Rome's entire budget out of his personal funds with plenty left over.

So in essence it was a "gilded age" similar to todays, the wealth<->power inversion notwithstanding.