Breast augmentation and reduction
A breast reduction for your larger breast and a breast implant for the other might suffice. This would even out their size asymmetry. If your overly large breast is causing you pain or other health issues, then it may be covered by OHIP.
Asymmetrical breasts with one significantly large and the other too small can definitely be corrected with a reduction on the larger and an implant on the smaller. This does, however, require significant expertise, so make sure you choose a board certified plastic surgeon with extensive experience with breast remodeling.
You would have to check with your insurance company regarding coverage. There are many variations to correct the asymmetry and an in office consult wold be helpful to review this options after a careful exam.