The most likely reason for this is that the zip file has been compressed using a different algorithm, or compression level, than the one used by the other programs.
The Finder uses the "data deflate" algorithm for zip files, while most other programs use "store" or "deflate". "Data deflate" is a variant of the "deflate" algorithm that includes a small bit of extra information in the compressed file. This extra information helps the Finder to uncompress the file faster, but it also makes the file a tiny bit larger.
So, if you compress a file using the Finder and then uncompress it using another program, the file will be slightly smaller. Conversely, if you compress a file using another program and then uncompress it using the Finder, the file will be slightly larger.