Why do my breasts look so full if they are a small size?

I am 23 years old, I have a slim build and I am a little short in height. I am a size 28D but somehow my breasts look fuller than usual.

I lent one of my bras to a friend who is the same size for comparison but her breasts don't look as full as mine.

Is there any explanation for this?
Does this happen to any other girl?

Why do my breasts look so full if they are a small size?
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