In my opinion, the sun is not change the size, but it is because of during the day we do not notice the sun size according to its sunlight. we usually look at the sun in the early morning and in the evening because the sunlight is weaker. actually it is the same size. you should probably use the film and look at the sun through the film, you will see that it has the same size. 
may be you may alreday know the asnwer, tell me when we meet again