Mission Control groups previews of windows from the same app. By hovering the cursor over a these group and scrolling up, you can see that the group showing more detail in those behind the main window. With the following command you enable to see a full size preview of the contents.
defaults write com.apple.dock expose-cluster-scale -float 1
To make the command take effect, you need to restart the Dock.
Revert to the default setting:
defaults delete com.apple.dock expose-cluster-scale
Restart the Dock