Can you please tell me if it's natural to have one breast bigger than the other?

Yes statistically L: The left is statistically larger than the right breast but this is typically less than a 10% difference in size and usually not noticed by the average individual under normal circumstances. On occasion a larger difference may risk and benefit from reduction (lipo/excision) of the larger and/or augmentation of the smaller with implants (saline/silicone) or with fat transfer (see image for example).