This is true that the stars are much bigger in size than the planets. But this is not the reason why the stars twinkle but the planets do not. Actually the planets do not have their own light to twinkle. As the stars have their own light, and as they are very much for away from us, the light coming from the stars has to travel a very long distance and for a very long time to arrive us. And so the light coming from the stars is found to change their intensities continuously and hence the stars are seen to be twinkling.