When you're catching a Pokémon, there is a coloured circle inside the white circle. This circle goes from green to yellow to red. I'm guessing this colour represents the difficulty of capturing that Pokémon because yellow-red Pokémon seem to break free a lot.
However that circle varies in size and goes from being overlaid over the white circle all the way down to tiny little circle I can barely see.
What does this size actually mean? Is it distance that your Pokéball was from the Pokémon or something else?