Pardon me for any snarkiness that may be apparent in writing (I promise it's not intended), but...says who? Following Caesar's successful proconsulship there was the civil war, when the size and power of the Roman military skyrocketed. Following Sulla's law granting promagistrates imperium in the provinces you have a gigantic upturn in the number of legions and the abilities of them, which was not resolved until Augustus took control of Antony's surrendered legions after Actium (and after the Nile as well). That's decades after Caesar's proconsulship. And if anything Roman military power only increases under Augustus, because even though he imposed a limit on the number of legions that could be on-duty at any one time, with it came a change in the way that military power was exercised. The downsizing of the army meant that permanent frontier garrisons became vastly more common, particularly along the Rhine and Danube. As opposed to the more general occupation of provinces by promagisterial armies after Sulla we see a lessening of military presence throughout most of the Imperial provinces (Africa being the only Senatorial province with a legionary force), but an increased presence on the frontiers (c.f. the military actions in northern Macedonia under Piso's proconsulship. Additionally, take a look at the number of legions stationed on the Danube, the same frontier as Piso's, under Augustus). And with the downsizing of the army it's usually argued that the quality of soldiers increased--the armies of the Principate were far more professional than those of promagisterial governors in the 1st Century, B.C.

u wot mate?

Xenophon has already covered this pretty well but this is a point that bears repeating: The Roman military did not enter into a period of decline after the death of Caesar.

It was after the death of Caesar, during the reign of his adopted heir Augustus that Rome first developed a standing, professional army. With the possible exception of the Assyrians this was the first such army in the history of the world. The full-time nature of service in the army allowed for soldiers to look at it as a career (the minimum term of service for an enlisted man was 25 years) and as such saw levels of professionalism improve massively. During the Republic armies were raised at the start of each campaign, this meant that these armies were extremely inexperienced and unwieldy at first (see the disaster at Cannae for what happens when an army that has been hastily scrambled together faces off against a highly experienced, cohesive army). The massive edge that was held by experienced armies can also be seen in the success of the campaigns of Caesar and Scipio.

In addition to the men and officers (the centurions at least) becoming more professional the change to a standing army under the empire also greatly furthered power-projection. During the Republic the raising and deployment of armies was ad-hoc, with local governors often using armies more to further their own personal plans and ambitions than to defend Rome. While still occasionally (and bloodily) distracted by Civil Wars the army of the Empire was able to focus much more closely on defending the border and fighting Rome's enemies.

Adrian Goldsworthy has written extensively on the military of this period. imo the best starting place for understanding the army of the Roman Empire would be his 'The Complete Roman Army'.