A Qt app with the networking library and the GUI library gets to about 10MB; that is, without code.
For people with Internet connection like mine, that is a bit inconvenient; specially when it comes to a program that is rarely used (i.e. used once and then left.)
I tried to use LZMA and LZMA2 compressions to compress the exe files, and it did nothing.
I would like to make the code itself smaller, is there a way to do that? How much can I go down in size? At least when it comes to the networking library.
PS: I wouldn't like to use UPX or such applications.
You could try statically linking; this may be able to discard unused code from the library. Please note, however, that this may impact your obligations under Qt's open source licenses; distributing a closed-source (generally, non-LGPL) application statically linked against a LGPL-licensed library brings with it additional obligations when compared to the dynamic-linking case. Of course, if your application is GPL or LGPL licensed itself, this is no problem.