Do you feel itching, burning, and stinging in skin folds?
Do you have a red flat rash with sharp scalloped edges with smaller patches of similar appearing rash nearby?
Have you noticed malodorous drainage?
Is this appearance always aggravated with sweating?

No, I haven't had any itching, drainage or discomfort at all. And no rashes either. I do like to go without a bra at home and have fairly large breasts, so I wonder if they could be pressure sores. An it is usually, but not always aggrivated by sweating.

What you are experiencing is known as intertrigo (inter, between, trigo friction), an irritation of touching skin surfaces in body fold regions, like under the breasts.
Red or brown, slightly elevated lesions with clear borders are typical of intertrigo.
It's common for women with heavy breasts, and can be worsened by any conditions causing increased heat, wetness, and friction.
Irritated skin along with dark, warm, and moist environment under the breasts is suitable for complications of intertrigo with yeast or bacterial infections.
Women with high or border-line blood sugar levels are more prone to this condition.
First you need to dry this area by applying some powder and check it with a dermatologist for additional infection with yeast (personally, I don't think your intertrigo is already complicated with yeast or bacteria).
If there is a yeast infection, Candida, you should need to apply some antifungal cream.

Best wishes!
Marija

Thanks for your response. However, I've looked at several examples of intertrigo online and they look nothing like what I have. These spots are about the size of a pencil eraser, in the exact same location on each breast and look more black than brown, and are not in the fold of skin under the breasts but about an inch higher.

Have you check these spots with your dermatologist?