Two photoshop files with a similar number of layers but one is twice the size of the other

I have two bank accounts with the same number of deposits  one account is bigger than the other I see nothing wrong  with that. All layers are not the same size in all document and documents canvas sizes can differ. All Smart Object layers have and associated layer transform. So Smart object dragged or place ins a doe object may be lint or embedded will be the same size the Layers pixels can be transformed to a different size that the object. You can not judge document  size by the number of layers have different amount of data in them. And layer can be any size they need not be canvas size.

As it appears to be my role to be hypersensitive to metadata bloat :]

https://prepression.blogspot.com/2017/06/metadata-bloat-photoshopdocumentancestors.html

Assuming you mean the file size on disk, factors affecting size:

The size and content of smart objects

The degree of compression used when saving the file (Preferences > File handling)

Image content , if compresssion is used.

Whether maximise compatibility is set (Preferences > File Handling)

If you mean the size in RAM shown at the bottom left of the Photoshop window, then that treats each smart object as a single pixel layer at the document size , no matter the number of layers and pixel size of the SO contents. So tends to under-report, compared to filesize, when smart objects are involved.

Dave