Physics.Arcade.Body#syncBounds

If true the Body will check itself against the Sprite.getBounds() dimensions and adjust its width and height accordingly.
If false it will compare its dimensions against the Sprite scale instead, and adjust its width height if the scale has changed.
Typically you would need to enable syncBounds if your sprite is the child of a responsive display object such as a FlexLayer,
or in any situation where the Sprite scale doesn't change, but its parents scale is effecting the dimensions regardless.