[roger64]

This question arises from a casual look I gave to a Sigil report on the EPUB where I had added an index: I noticed that the total size of all text files was 473k and that the size of the index file was 364k. Really? an index whose size is 3/4 of the text files? I wanted to know more about it.

Here is the answer: basicallly, as Kevin said, it's all about compressed and uncompressed sizes. But if you do not check it, you do not know it...

Without any index at all,
the compressed EPUB size is 2092 k
the uncompressed EPUB size is 2608 k
But once indexed,
the compressed EPUB size increased to 2129 k (a + 37 k)
the uncompressed EPUB size increased to 2978 k (a + 370 k)

It appears from these figures that index data are particularly sensitive to compression (from 37 k compressed size to 370 k overall* uncompressed size).

@Doitsu
Other information: the compressed size of the 1657 index tags was about 7 k but this figure only adds to the text files, not to the index.

* saying "overall", I count the 364 k of the index.xhtml file plus the 7 k text data above.
The data left in the window of the index editor belong to Sigil.