If you are using the BigPicture push notification template on Android, we recommend you to choose the following ratio for your image:  1.8 (width) X 1 (height).

But, the ratio 1.8 is the adviced ratio to have a correct display on a large sample of devices.

The image would be reduce only on the height. That's why it is advice to center the information on the image.

Do not forget : more the push content text ligns more the image will be reduce.