Size of a library and the executable

Question

I have a static library *.lib created using MSVC on windows. The size of library is say 70KB. Then I have an application which links this library. But now the size of the final

Answer 1

A static library has to contain every symbol defined in its source code, because it might get linked into an executable which needs just that specific symbol. But once it is linked into an executable, we know exactly which symbols end up being used, and which ones don't. So the linker can trivially remove unused code, trimming the file size by a lot. Similarly, any duplicate symbols (anything that's defined in both the static library and the executable it's linked into gets merged into a single instance.