what changed so much that the program doubled in size from v 3.1 to 3.5 ?

[leao-nardo]

i had the portable version 64bit and the exe inside the unzipped folder in v3.1.2 was 7mb i downloaded the current v3.5.2 and its 15MB

[GitHubRulesOK]

Much of the increase comes from "Feature" requests (Theming Annotation etc.) and some from the MuPDF size increase of their improvements, but compare the portable run time versions
3.5.2 16,065,496 bytes (compacted installer 8,246,744 bytes)
current pre-release is 19,188,096 bytes (installer is 10,350,976 bytes)
so not double

[kjk]

A big contributor is more bundled fonts, for more robust rendering of Arabic, chinese etc. text in pdf and epub files. Font files, especially for languages with many characters, are big.

Also, more code to support more image formats and better epub rendering. ePub is basically html so to render it you need to implement a mini web browser.