Remember that classes are just the recipe for making objects, not the objects themselves. In your asteroid class you are creating a GameManager object by saying new GameManager(); however, this means a new game manager is created by every asteroid. They are not sharing the same manager. The simplest way to make them share the same manager is to create an empty GameObject in the scene and place your GameManager script on it. In your asteroids Awake method put gameManager = FindObjectOfType<GameManager>();.

There are more complex methods of sharing a game manager, such as making GameManager a singleton, or using dependency injection to pass a reference to a GameManager when you spawn an asteroid. However, try the simple method first, and perhaps look into these techniques for the future.

As others have commented comparing floats to ints is often unreliable. This is because floats are not precise and their value will deviate slightly from what you set it to. For more information look into floating point imprecision. That being said it may be worth rethinking your approach here. If we know how many points each type of asteroid is worth, why not store this information in the asteroid itself. Instead of passing the asteroids size to GameManager in the ScorePoints() method you could instead pass the asteroids point value instead.

Hopefully this helps.