Anime / Manga - 5 kilograms

For all you Americans that's a little over 11 pounds, or a little under 1 and a half gallons of milk.

He's not compressing, he is lifting. And by lifting her breasts he is reducing the weight they contribute due to the fact that they are no longer pulling down on her frame. It would be like if I tied a sandbag to you chest, put you on a scale, and then lifted the sandbag a little.

Does that help?