If you compare Disk Usage Analyser and System Monitor (as snap mounts them like separate devices), the numbers don't actually add up.
This could mean Disk Usage Analyser is also counting symbolic links, which are files that "soft link" to another file, but are treated like they are a separate file:
So in this case, these two 300 kB files will appear to programs as 600 kB, but physically these two files only use 300 kB on the disk.
Just a theory, I'm no expert in snaps. I'm aware snaps in general can be larger in size then their deb package counterparts as they ship their own a copy of the libraries they use, as opposed to using installed libraries system-wide. (Pro = self contained, no conflicts. Con = larger size, a snap could ship an outdated library)