Have a small chest (aero boobs) as well and wear the Oiselle bras and have never had an issue and my HR monitor strap is super loose, I just tuck it in under my bra so it's above the bottom bra band. I bought a bra a few years back where you just snap in your HR monitor but the sensors on the bra were crap and I could never get it to pick up the signal.

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Elisha
"Triathlon doesn't build character. It reveals it."