How does the southeast border of present-day China compare with that of China during the Qin dynasty

Interesting question!

The ancient Qin dynasty was much smaller in size compared to the modern People's Republic of China.

Many parts of modern China's south-eastern provinces such as Guangdong and Fujian were not part of Qin dynasty.

However, generally the Qin Dynasty held sway over an An empire which could fit very well in modern China's eastern territories