Well, I tell what I see :)
Compiling kitty with settings below produces this big
/usr/lib64/kitty/kitty/fast_data_types.so:
* Without any LTO-related flags: 4.52 MB
* With -flto: 4.30 MB
* With -flto -ffat-lto-objects: 4.79 MB
Well, I did not run compilation multiple times but don't think it will
change much.