How did Charlemagne create a large empire?

Charlemagne mostly enlarged his empire through military conquest, most notably through a long and successful series of campaigns against the Saxons, and his conquest of Bavaria. From an administrative standpoint, he gave his authority increased underpinning by assisting the Pope against riotous Roman nobles and subjugating the Lombards, actions which earned him the title of Holy Roman Emperor. He adopted a system of standard weights and measures throughout his empire, including a new system of coinage. Charlemagne projected imperial authority by moving his court throughout the empire, and delegating administrative duties to local appointees known as counts. He communicated with these counts through messages sent by officials known as missi dominici. Additionally, he divided the empire into larger administrative regions run by governors. Finally, Charlemagne reformed the legal code, compiling Frankish laws with Roman laws.