My problems when underdressed are not so much boob volume as such, but my nipples and bra straps.
My pumping regime has led me to require the support of a bra full time. I have useful wobbly breast volume and large nipples which hang heavy and which protrude a lot. I am on the border of a B & C cup, and I need a padded bra to hide the nipples.
This produces significant booby bumps, but I find that if they are smooth and under a fairly loose shirt, then they don't attract much notice. Many non TG men are quite heavily breasted, and seem to be OK without support!
Bra straps are much harder. It's a combinsation of the bra colour and the strap buckles. A white bra shows under almost any shirt, whereas, surprisingly, a dark red one is difficult to spot.
The bra strap buckles show through under anything than the thickest shirt. I am sure that I get noticed by women quite regularly!
At least I make sure I don't suffer a twisted or dropped strap, which are dead giveaways.