I conclude from above comments and further tests, that the problem is a bug in the SDK: when creating a WebView via XML layout, the activity is passed as the context for the WebView, not the application context. When finishing the activity, the WebView still keeps references to the activity, therefore the activity doesn’t get removed from the memory.
I filed a bug report for that , see the link in the comment above.
webView = new WebView(getApplicationContext());
Note that this workaround only works for certain use cases, i.e. if you just need to display html in a webview, without any href-links nor links to dialogs, etc. See the comments below.