JPG images may or may not include resolution data (physical height by width). If this data is not embedded in the file, an app may open it at a default resolution.
The solution is to ensure that you have saved this resolution data in your JPG file with an application that can save resolution data (not all do).
An alternate solution is to use another image format that saves resolution data. TIFF is always nice. You can get pretty good lossless compression from LZW or ZIP in the TIFF format. These types of compression do not harm the image quality like JPG does.