[oriesdan]

Not sure what you mean with external sources, but I have seen nothing but user generated content in there (but I haven't read all wikipedia articles, obviously).

A few things to note, though:

1/ it's not pure text content, it's html content, this has a significant overhead

2/ a zim file is not just compressed content, but also huge indexes referencing where is which content. You look for your article's title in the reference table, you find the position of your article in the file and you decompress just that part. This is what allows for selective decompression without decompressing the whole content.