Those who can help you are going to need a lot more info than what you give.
What exact version of the OS? Version of Photoshop? Where do your original files come from? How are you saving and in what format? What exact image dimensions (so many pixels wide by how many pixels high? What color space?
How are you sharpening, and at what stage? Did you recreate the Windows action from scratch on your Mac?
If the same image looks different on your Windows laptop than on your Mac laptop, what browser(s) are you using to view them in each platform?
Sample files (or links to them?)
Sorry, my mind reading and clairvoyant hats are still at the cleaners.
Post screenshots and links to the photos. With those Retina MBPs there could be any number of factors at play like your browser simply scaling the pages by a factor of two...
…With those Retina MBPs there could be any number of factors at play like your browser simply scaling the pages by a factor of two…
That's the thought that entered my mind.