when you upload a image to the gallery it is re sized if needed by gd2 in your case as I recall.
Control Panel » Gallery Settings
for example
Full image max width/height in pixels. 1200
If either the width or height exceed this value then the larger sized property will be scaled to this, with the other being proportionally scaled.
so if a picture exceeds 1200 it will be re sized down to 1200.
same goes for thumbs and medium.
so it depends on the display used if it is capable of displaying it as is or the devices browser like safari may do some resizing.
as far as the mac it could be your display setting . Try setting it to a higher resolution. I use 1280X1084 for my display on my PC
