No matter what app you're using, Android provides the same Share menu with the same list of options in it. You want to share to Pinterest from within Dolphin browser? That's no problem, because the list is independent of the app, and the Dolphin developers don't have to think of Pinterest when they write their app.
On iOS, however, every app has its own unique Share menu. So if the makers of your favorite photo-gallery app haven't thought to include Google in their Share menu, you can't share to Google from within that app. For example, neither Safari nor the iOS Photos app can share to Google, Pinterest or any of 100 other services.