Why did the Qin Dynasty send 300,000 troops to the Great Wall to fight Xiongnu, but 500,000 troops to Guangxi?

Why did the Qin Dynasty send 300,000 troops to the Great Wall to fight Xiongnu, but 500,000 troops to Guangxi? This is really a puzzling question. According to people's understanding of that history, it is generally believed that:

At that time, Qin Jun was mainly composed of four parts: the Southern Army, the Northern Army, the Central Army and the Lishan Criminal Army organized by Zhang Han. According to historical records, the 300,000 Mount Li Corps led by Zhang Han was mainly used to quell the civil strife in the Qin Dynasty. The northern army guards the Great Wall, and the role of the central guard is self-evident, while the southern army is stationed in Lingnan area, with a depth of 500,000. If that's all, in the deployment of the four legions of the Qin Dynasty, it still reflects the importance that the north is more important than the south.

But I still don't seem to want to say this, because at least the Mount Li Corps in Zhang Han was originally just prisoners and workers who built the mausoleum of the First Qin Emperor. Although it later became the main force to suppress chaos, it was only a temporary "positive" reserve force at best and could not be regarded as a regular army. So, didn't the military deployment of the Qin dynasty become more important than that of the north? Isn't this contrary to the background of Xiongnu's invasion to the north and the "rise" of rebels in the Middle East?

Faced with this situation, some people put forward the following explanation: At that time, Qin Jun went south, although it won a small victory for the first time, but with the passage of time, the war situation became more and more unfavorable. In order to save the defeat, the Qin Dynasty had to use "refueling tactics". Over time, the total troops invested in the southern expedition reached 500 thousand! Second, the Southern Legion of the Qin Dynasty was weak, especially compared with the elite Northern Legion guarding the Great Wall. So, at best, it's quantity versus quality.

There is another saying: At that time, Qin Jun went south to Baiyue, and many people in Qin Jun were unwilling to go because of the remote geographical location and uncomfortable climate, and the local people stubbornly resisted. This incident angered the first emperor, so Qin Shihuang not only forced the army to March south, but also asked the officers and men to bring their families and non-military personnel to do odd jobs. According to such miscellaneous statistics, there are about 500 thousand in total.

Finally, there is another point of view, that is, does the Southern Legion of the Qin Dynasty really exist? Or is there really a "luxury lineup" of 500,000 elite troops? According to historical records, there were three commanders in South Qin Jun. The first is Tu Youyou, whose official position is only Commander-in-Chief of the Army. The second is Ren Tao, who was named Nanhai Wei only after Lingnan was pacified. The third, Zhao Tuo, is just the county magistrate of Xialongchuan County in Nanhai County.

Imagine: a commander-in-chief commanding 500,000 troops may be just a small county magistrate? Moreover, if Tu Youyou, the first commander-in-chief of the Southern Corps, really had 500,000 troops, how could he be easily defeated by several bandits in Lingnan? This is a big joke!

Thus, if history can be restored, people who hold this view believe that the Southern Legion of the Qin Dynasty at that time, in terms of its scale, would not exceed 50,000 troops, not the legendary 500,000!