Android -- Why my app uses about 40MB Cached background process?

I'm starting a new app with minSdkVersion="14" and targetSdkVersion="17". It contains a viewpager with 6 pages. There is 3 webviews and 3 others views.

When i push my app to background by clicking on back or home button, it uses about 40MB in "cached background process" and i don't understand why.

This is an example of one of my webview :

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.RelativeLayout;

public class Presentation extends Fragment {
    boolean isOption = false;
    RelativeLayout main = null;
    WebView web_main = null;

    public Presentation () {

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        main = (RelativeLayout) inflater.inflate(R.layout.webview,  container, false);
        return main;

    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onActivityCreated(savedInstanceState);
        web_main = new WebView(getActivity().getApplicationContext());
        web_main.setWebViewClient(new WebViewClient());
        web_main.getSettings().setAppCacheEnabled(false);
        web_main.loadUrl("file:///android_asset/main.html");
        main.removeAllViews();
        main.addView(web_main);
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.i(getClass().getName(), "[OnDestroy]");
        main.removeAllViews();
        web_main.destroy();
        main = null;
        web_main = null;
        System.gc();
    }

}

I followed several tutorials and answers but there are no effect on cached background process This is my ma