I just did a real test between a full build, and a build using prebuilt cocos libs.
For the record, I am using the libs build in the full build (just copied to the prebuilt folder) for my prebuilt build.
This means that both builds use the exact same cocos .a files.
My game so is 2.73 MB for the full build, and 10.0 MB for the prebuilt build.
My APK package is 6.52 MB for the full build, and 9.48 MB for the prebuilt build (made with Eclipse).
EDIT: Some buildinfo, I'm only building armeabi-v7a and with these APP_CPPFLAGS:
APP_CPPFLAGS := -frtti -fexceptions -std=c++11 -fsigned-char -Wno-extern-c-compat -DCC_USE_PHYSICS=0 -DCC_USE_3D_PHYSICS=0 -DCC_ENABLE_BULLET_INTEGRATION=0 -DCC_ENABLE_BOX2D_INTEGRATION=0 -DCC_ENABLE_CHIPMUNK_INTEGRATION=0 -DCC_USE_NAVMESH=0 -DCC_USE_TIFF=0 -DCC_USE_WEBP=0 -DCC_ENABLE_SCRIPT_BINDING=0