Why movieclip size does not bloat up when using getSize?

Hi guys, this puzzles me.

I added a big image 1.6megabytes into a movieclip,
and use getSize to check the movieclip size, it shows 456 (bytes??)

Then I added a 45kilobyte icon into a movieclip,
and use getSize, it traces 456

If I check against the size of the children
its always 256. Why?