Why do males talk to my chest?

I am very ordinary looking and over the years I have had male acquaintances. I have found that some of them in conversation look at my chest then my face and the conversation goes with his eyes going up and down. Do men realise they are doing this? Is it an instinct they don't know about? Is it a sign that they fancy me or is it a fascination with any ? However most of them never asked me to "date" or wanted any relationship. men what are you thinking? I don't wear skimpy clothes and my chest does not appear the biggest.