What size does NSIS add to the compressed data?
From NSIS Wiki
NSIS is the smallest win32 installer system available. With the default options, it has an overhead of only 34 KB over the compressed data.
If you have added graphics to the interface, have very long scripts or if you are using plug-ins, your installer will be a little larger, but all graphics and plug-ins will also be compressed.
The graphics and scripts code of a standard Modern UI installer/uninstaller will add about 25 KB (but you can reduce this to about 5 KB if you don't use icons with lots of high-color image formats).