[Mp4-tech] Size of first I frame

[Damodaran, Usha (UMKC-Student)]

Hi,
I have a question regarding the size of the very first I frame in an MPEG4 clip. In three test clips I have seen - Akiyo_qcif, Foreman_qcif and Mobile_qcif - the first I frame is the largest (much larger than the subsequent I frames). Why is it so? Isn't it true that each I frame is independent of the others? So, my idea was that all the I frames should have approximately the same sizes or depending on the scene would vary a little, but here consistently the first I frame has the maximum size. Is there any additional information in the first I frame? Similarly the first P and B frames are the largest compared to the other P and B frames in the whole clip.
Please let me know. I would really appreciate your replies.
Thank you,
Usha