Every mausoleum is named after one very large tomb.
The word comes from Greek Mausoleion, the name of a marble tomb at Halicarnassus in Asia Minor, today's Bodrum in Turkey. Etymonline says Artemisia, the wife and sister of the ruler Mausolos, had it built, and dates it to 353 BCE. Latin borrowed the name and English had it in the early 1400s, meaning "magnificent tomb."
Mausolus was a satrap in the Achaemenid Persian Empire, according to Wikipedia, and he began planning the tomb. He died in 353 BC, and Artemisia II carried on the project. The tomb was built between 353 and 351 BC, and she ruled alone for about two years before her own death. The Greek architects were Satyros of Paros and Pythius of Priene.
The building was large. Wikipedia gives its height as about 45 meters, with sculptural reliefs on all four sides, each side carved by a different leading sculptor: Leochares, Bryaxis, Scopas and Timotheus. Antipater of Sidon put it on his list of the Seven Wonders of the Ancient World.
It did not last forever. Wikipedia says successive earthquakes damaged it from the 12th to the 15th century. The Knights of St John then used its stones to build and fortify Bodrum Castle. Etymonline says the tomb was counted among the Seven Wonders until an earthquake destroyed it in the Middle Ages.
Around 1600 the name stopped meaning one monument. Etymonline records the broadened sense of any stately burial place, often one holding several tombs, from that time. The adjective is mausolean.
So a satrap's name now stands for every large tomb, whether or not it is as grand as his.