Symptom
Even though the Rescale the image button is switched on, the image cannot be enlarged in the player.
Cause
Your uploaded image needs to be more than 500px wide, otherwise the user will not get the option to enlarge it in the player, even if this setting is checked.
When you check the option to Rescale the image, and it is larger than 300px, it will be automatically resized to 300px wide in the backoffice, and then you cannot change the size anymore.
Solution
Use an image that is more than 500px wide.