Hi,
The profuse sweating in the area may lead to growth of bacteria which may contribute to the odor.
Is there any associated itchiness or redness in the area ?
Any associated scaling which could suggest an associated fungal infection?
After taking a bath, pat the area under the breasts dry and spray an antiperspirant on the area .An antiperspirant spray may be better compared to roll ons which may further irritate the area upon contact. At night, you may apply Vitamin E cream (not lotion) thinly over the area to help soothe and moisturize the skin.Wear brassieres with good support and breathable material. Sport bras usually give good support and they are usually made of light and less irritating materials. Choose underwears with hypoallergenic materials .There are new products that have this feature.
Avoid places which can make you sweat a lot. Also try to change your brassiere and shirt each time you experience excessive sweating. This will help prevent further skin irritations which may lead to other skin problems.
I am assuming you have large breast; but even if not, have you tried using your deodarant there. I have large breast & it helps to wear a cotton breathable bra & an ample amount of anti-perspirant/deodarant there as well.