What are the chances of a malignant breast lump decreasing?

Not likely: All breast lumps need evaluation. Don't assume something's ok because it seems to have decreased in size. Malignant lumps usually increase in size, but other factors may make them seem to decrease in size temporarily.

Biopsy?: Have you had a biopsy and been diagnosed with cancer? Cancerous lumps do not decrease, as my collegue replied in his answer. You could have a fluid filled cyst in the breast that can decrease in size and often times disappear altogether. I recommend you get evaluated by a physician to get a correct answer and diagnose your problem. Good luck!