I get that this is an attempt at a joke, and I'm sure "it's not that serious", but I swear I'm so sick of reading people talk about women's bodies, especially pregnant women's bodies. I feel like there was really no need for your comment, as it's basically just the OP observation, and it really just adds to an undercurrent of sexism/misogyny/objectification I'm seeing online. I doubt that was your intent, and I'm not sure if you personally care, but I just thought I'd mention in case it has any chance of changing the way you or someone else decides to talk about others.
If you want to talk about your own chest or what you do with it, feel free to, but talking about the chest of a work colleague in this context just comes off as gross to me, especially because it's relating to them being postpartum.