Among the formats:

Rocket
Mobipocket
MS EBook
Plain Text
Web Document
Embiid
MS Word 2002
hiebook
Adobe Acrobat
MS Wordpad
eBook Pack Experss

thx

Mobipocket developped a new compression algorithm that can give up to 10 % compression rate.
The algorithm you may have tested (included in Mobipocket Publisher) applies a standard compression (around 50% on text data).
Non text data (images, indexes, frames information, ...) are not compressed this way.