Android : NDK vs CMAKE optimizations

I recently switched my Android/Application mk file to a CMakeLists.txt file.

A good portion of the code is native in c/c++ (hence why cmake's debugging ability is required).

Now though, I am comparing the two builds, and the one built with ndk is much faster ...

I have added in the "-Os" flag to the

externalNativeBuild {
    cmake {
        arguements "-Os"
    } 
}

block inside the build.gradle file.

My question is what is the differences of compilation here ?? What about ndk-build and cmake is different that would cause this ??

I feel as if this is a dumb question with a quick answer