why different MovieClip Size?

I'm sorry. I can't write English very well.

but I think, you can understand attachment file sample, if you use Flash CS4 with ActionScript 3.0.

========== example ============

1. Same Button and Same Size on Stage.

2. First Button is only on Stage.

3. Second Button in MovieClip.

but Tow Button result different Size.

Why different Size? I can't understand this sample result.

Thanks for you.