If it still helps:
That’s a legacy behavior of JPG. JPG are “web” images (by default 72dpi) , so they do not store dpi settings. Use other formats (like png) which do lossless compressions and store dpi settings. In case you need to stick to jpg, just re-set the correct resolution in indesign/photoshop etc back to your setting (350 DPI, without resampling), you’ll get the expected size in millimeters (while the size in pixels remains the same).