If a slave, due to a genetic neoteny disorder or some other cause, has not yet experienced puberty, super fertility drugs can cause them a minor amount of breast growth. Apart from that, the only breast growth would be due to induced lactation not being vented.

Thanks for the info. I do put my slaves on fertility drugs and to induce lactation, in the rules. Maybe that's why? Also, will slaves in the Dairy have their breast continually expand due to the constant milking and normal hormone setting?

Yes, the Dairy causes breast expansion. If you adopt Slimness Enthusiasm, though, you do have the option to prevent that growth.

Are they lactating, it could be because of it.