The cups fit perfectly and comfortably. They're also slim and lightweight, so I don't notice them most of the day. These cups are pretty versatile and are not only used for collecting milk. For instance, I prefer to use these cups instead of nursing pads, specially on days when I'm over-producing. The silicone also feels like it's good quality and it's safer than plastic. They are slim and look translucent so I don't have to worry about them being noticeable through my clothes. I do wear a slightly bigger bra size so that I can fit them comfortably. I'm not sure if these are One Size Fits All, because they're not adjustable.