Does AirDrop alter images in any way to reduce size for faster transfer or the "Preparing" is preparing the package for tranfer, involving no image resize/compression?
When it was originally launched for iOS (iOS 7), AirDrop occasionally down-sampled photos and videos for no apparent reason. However, since a later release (possibly iOS 8), I have never seen AirDropped media in a size/quality other than the original.
When sharing photos and videos, “Preparing” indicates that the Photos app is downloading the full-resolution version from iCloud Photo Library. You will see this step regardless of the channel through which you’re sharing (including email and iMessage).
You should be able to verify after the fact that your device now holds a larger-file version of your photo or video (eg. by opening Image Capture or checking your storage used). See this answer for further details.
There are notable exceptions:
- Slo-mo videos are finalized and reencoded, which removes their ability to have their “slow” portion adjusted.
- Edited photos and videos need to be reencoded (to include those edits), so the quality may be slightly different vs. what was originally shot. However the resolution will remain unchanged.
- It is unclear what quality setting Photos for macOS uses when sharing via AirDrop. You can enforce a certain quality level by first exporting the photo to your desktop, and AirDropping that file, rather than AirDropping straight from Photos. You'll probably want to use High quality and Full Size for edited photos, and Unmodified Original for unedited photos. See this answer for more details.