android - prebuild project apk size increased in target folder

Question

I try to run below prebuild project in aosp code. I found the apk size increased about 18MB after copied to target folder:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE        := com.sample.test
LOCAL_MODULE_OWNER  := personal
LOCAL_MODULE_TAGS   := optional
LOCAL_MODULE_CLASS  := APPS
LOCAL_CERTIFICATE   := platform
LOCAL_MODULE_SUFFIX := .apk
LOCAL_PRIVATE_PLATFORM_APIS := true
LOCAL_SRC_FILES     := ./test.apk
LOCAL_PRODUCT_MODULE := true
LOCAL_PRIVILEGED_MODULE := true

include $(BUILD_PREBUILT)

Original test.apk and output apk size (14M to 32M):

-rwxrwxr-x 1 me 14M Feb 23 10:42 mytest/test.apk
-rw-rw-r-- 1 me 32M Feb 23 10:43 com.sample.test/com.sample.test.apk

What's the reason the prebuilt apk size increased so much?