Since the GameObjects that were created with the Square sprite placeholders had a polygon Collider added to them, although the Collider was created to match the geometric Square shape of the Square sprite placeholders, it can be edited and made a different shape.
In this recipe, you replaced the Square sprite with the Enemy Bug sprite image, then edited the Polygon Collider 2D component so that it matches the oval shape of the outline of the Enemy Bug image. By removing the GameObjects with the Square polygon Colliders, and then duplicating the one containing the oval bug-shaped polygon, all the falling GameObjects collided to match the visible shape of the Enemy Bug images.
This approach can be used to create polygon Colliders to approximate the shape of any image outline you might wish to use in your game.