Every photo imported into Day One is converted to JPEG format and resized to a maximum resolution of 2100 x 2100 pixels. The aspect ratio is maintained. We resize photos for more efficient sync and storage. At this size the average photo is about 700KB.
When you take a photo using Day One on your iPhone or iPad, by default the original, full-resolution image is not saved to your Camera Roll. You can change this behavior in the Day One Settings > Advanced panel.