A slight change in file size on disk can be normal when moving a file between storage with different formatting and cluster size but a reduction of half if quite a lot. I have tested on my iPhone 5s and iPad mini and I am not seeing this file size reduction.
Can you provide some more info? That will help us debug.
- What is the exact model of phone or tablet you are using?
- What is the OS version?
- How are you transferring the pictures? Manual transfer? Auto backup?
- Could you post exact steps you are taking when transferring the files?
- Could you shoot a video of the issue and post a link?
- Could you share some sample files before and after the transfer?
- What is the source of the pictures? Were they shot on the host device or are they from some other source?
- Do you have any other apps that have access to the camera roll?
Any additional information you can provide will help us narrow down what may be causing the issue.