The result of the decrease in population, loss of

The demise of the Roman Empire cannot be attributed to one cause alone. Instead, it was the result of the decrease in population, loss of land, and deception. One of the things that played a significant role in speeding, however, was the expansion of its empire. At its peak under Emperor Augustus, the entire population of the Roman empire was in estimate about 50 to 90 million. As a result of the large population, it made it difficult for emperors to control their empire, which in turn created disunity among it. Disunity and unsteadiness weakened the Roman military and left them defenseless against invaders. 
 Second, overexpansion destabilized the government and its currency. Emperors who needed to fund the vast empire made coins with less valuable metal in an attempt to provide the required income. Still, the resulting inflation made it difficult for armies and officers to maintain the cost of the necessary supplies, leaving them vulnerable. Instability in the government also resulted from driven men that competed for the emperorship causing wars, turmoil, and disputes. Civil wars used the available military assets needed to defend from an outside assault. 
Another minor reason was what might be viewed as moral defilement. The Romans put time and money in Gladiatorial battles and, “Emperors like Caligula and Nero became infamous for wasting money on lavish parties where guests drank and ate until they became sick.” Fall of the Roman Empire. Retrieved from https://www.rome.info/history/empire/fall/. 
Another factor was the loss of several strategic positions. For example, when Rome lost several of its territories in northern Africa in 439 AD, and that left its coast along the Mediterranean Sea vulnerable. Additionally, The Western Empire lost some of its wealthy lands to the Eastern Empire when they split 286 CE. 
The main reason for the fall of Rome was its size, which made it difficult for Emperors to control. The empire turned out to be too big to protect, finance and manage.  Rome’s territory was too big to defend because its outskirts reached out more than four thousand five hundred miles. The suburbs had turned out to be too broad for the military to control incoming assaults. Rulers attempted to fund-raise by expanding charges, however, without the cash produced by conquest, they were unable to renew the empire’s treasury. Third, with an area extending from northern Europe to the Middle East, one man alone couldn’t oversee the whole Roman Empire. Pronouncements and laws were sent to the governors of an area, but the emperor couldn’t ensure that they were followed after. This extended the disunity in the domain amongst individuals and the legislature in Rome. The large size of the Roman Empire kept rulers from funding, defending and ruling over the empire, making the shortcomings that enabled roaming clans to attack.