Improved natural attack is size dependent. Form of the Dragon (level 1) is a spell that causes you to "become a Medium chromatic or metallic dragon" - which you chose at first level, and cannot change. 'Medium' being the key word. Where level 2 makes you Large, and level 3 makes you Huge.
Here is the language of your ability, found at:
Bloodline Powers: The power of dragons flows through you and manifests in a number of ways. At 1st level, you must select one of the chromatic or metallic dragon types. This choice cannot be changed. A number of your abilities grant resistances and deal damage based on your dragon type, as noted on Table: Draconic Bloodline Breath Weapons.
Claws (Su): Starting at 1st level, you can grow claws as a free action. These claws are treated as natural weapons, allowing you to make two claw attacks as a full attack action using your full base attack bonus. Each of these attacks deals 1d4 points of damage plus your Strength modifier (1d3 if you are Small). At 5th level, these claws are considered magic weapons for the purpose of overcoming DR. At 7th level, the damage increases by one step to 1d6 points of damage (1d4 if you are Small). At 11th level, these claws deal an additional 1d6 points of damage of your energy type on a successful hit. You can use your claws for a number of rounds per day equal to 3 + your Charisma modifier. These rounds do not need to be consecutive.
Dragon Resistances (Ex): At 3rd level, you gain resist 5 against your energy type and a +1 natural armor bonus. At 9th level, your energy resistance increases to 10 and natural armor bonus increases to +2. At 15th level, your natural armor bonus increases to +4.
Breath Weapon (Su): At 9th level, you gain a breath weapon. This breath weapon deals 1d6 points of damage of your energy type per sorcerer level. Those caught in the area of the breath receive a Reflex save for half damage. The DC of this save is equal to 10 + 1/2 your sorcerer level + your Charisma modifier. The shape of the breath weapon depends on your dragon type (as indicated on the above chart). At 9th level, you can use this ability once per day. At 17th level, you can use this ability twice per day. At 20th level, you can use this ability three times per day.