What's the difference between the planning of Xi 'an today and the Great Wall of Tang Dynasty?

Today's Xi City is much bigger than Chang 'an City in Tang Dynasty. But Chang 'an in Tang Dynasty was much bigger than Xi in Ming and Qing Dynasties.

Look at the following two pictures. The first is the comparison between Chang 'an City in Tang Dynasty and Chang 'an City in Xi now (there is a bell tower at the upper right of Zhuquemen, which can be compared). The second picture shows Chang 'an City and Xi City in different dynasties.