We just talked about the fact that the Han maps are using different standards from the Mauryan ones. The Han map above depicts an area directly subject to a Weberian bureaucracy of taxation, administration and registration. If we use that standard to draw Mauryan maps, then the Mauryan Empire would have been restricted to the land directly under the king's control, or the region inside the eastern Gangetic plains and a few isolated cities outside of it. Even the truncated map of the Mauryan Empire only shows regions where Mauryan style inscriptions were found. If we use the presence of inscriptions and other artifacts as the standard to draw the Han, we have to include not just the counties of China proper (including North Korea and Vietnam), but Xinjiang and parts of Mongolia as well as we've found several Han inscriptions in these regions.Same types of differences appear in maps of many ancient empires:
Han China maps:
The territory of the second map looks over twice as large as in the first map.
The fact is that we really have imperfect information regarding the exact degree of territorial control of each ancient state as well as there is a difference between territorial claim and territory effectively controlled as explained by civfanatic explains.
The farther back in time we go the worse our knowledge regarding territorial control tends to be. Also I should point out that the as research is done more precise borders tend to be know and maps of territorial control tends to decrease in size as maps improve from speculative borders covering large landmasses to roughly where the borders might have really been. For instance, we know the Roman Empire's borders in 2nd century much better than the Islamic Caliphate borders in the 8th century, because the amount of research done for the first case is much larger than the second.