Left eye smaller than the right eye

Hi all,

My baby daughter is 4 months old now and i noticed when she was about 6 weeks old (although my mum said she noticed this earlier) that her left eye is smaller than her right eye / left eye appears more closed than the right, especially noticeable when she is upset or tired. I have been to th GP twice and because the eye lid doesn't cover her pupil he isnt concerned and can't really give an explanation for it, he doesnt think it's a problem.

Do any of you have experience with this? Or can tell me what this might be? 

Thanks in advance xxx