[thechao]

Ballpark figures based on the ram earth construction for TGE vs AOT would have the AOT wall be 5-10x the volume & mass of the TGW. The issue is labor — the Great Wall probably represents 20–100 million ma years of labor. The AoT wall probably has at most 100k man years of labor it could've pulled from. That'd mean it's labor-mass ratio is off by 1000–10000x.

[Izkata]

This gets into spoiler territory, but the walls in Attack on Titan weren't made with human labor. The first hint that something funky is going on was at the end of the first season finale, but we don't get the full story about the walls until much later.

[deaddodo]

It also was built when Marley was still very much a massive empire. So it wasn't limited to the manpower inside the walls. This is clear from how many of it's actual construction materials were used.