Size of the some photos from iPhone is 10x reduced in comparison to orignal files...

I bought Sandisk Wireless Flash to upload my photos from iPhone. To my big dissapointment size of some photos is reduced around 10x during the transfer and some are transfered ok. Firsty i thought it was because of HDR function or used color filters but it occured it wasn’t the case. What could be the problem? There is no visible rule for ok photos and not ok photos. I am using iOs 8.2.

I am not seeing any issue when i transfer pictures. There is a setting to compress video when transferred. I don’t think that would affect pictures but possibly it does. Try disabling that in settings and see if the issue still occurs. 

If the issue still occurs can you post a link to a picture in original size and a link to the picture after it was reduced during transfer to the WFD so we can test?