the whole episode was actually an outlier, the verse is consistently island level, any higher is inconsistent.
1. The seireitei isn't actually that big. The statements are from unreliable characters, and we have to use the visuals so the seireitei is 1 city block big.
2. Las noches can be pixel scaled to only be as big as a small town, so it's not that big either.
3. The soul society is clearly a smaller-than-normal city that's just the seireitei and some stuff around it because in the Color+ book there is niche quote that eludes to this being the case, so it trumps all the other stuff.
So in conclusion:
- Las noches destruction = inconsistent and never proven to be true
- the sokyoku is the power of a million zanpakuto, so clearly the sokyoku is 1 million x a shinigami's zanpakuto, and there are some which are barely wall level.