Why did he put his hand on my upper breast?

We’re both 15 and we flirt sometimes. I like him and he also acts different around me, joking with me, teasing me, looking in my eyes very often, smiling. When I look sad he looks very concerned. He touches my belongings. I feel so shy by his side and he seems to be amused by that.
Sometimes we walk the streets together in silence without saying a word. His guy friends often look at me. He tries to get my attention and always comes to my side of school.

The other day we were walking our way home feom the maths lesson and he suddenly made me stop. Like approached me and put his right hand on my upper chest side down my collarbone and left hand on my back near the upper waist. We stood like that for like 5-6 seconds without saying a word because I didn’t know what to say and he said then “You are so thin”

why did he do this out of sudden? What does that move mean?
Updates:
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I am not even that thin, I am skinny on the upper side but has got cute and grabbable butt and legs.
Why did he put his hand on my upper breast?
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