What's the difference between jpg and JPG on mac?

Why does the picture on top have 'jpg' capitalized, when they are all JPEG images? Is it because the file size is larger?

At a guess, one came directly from a camera, the others were saved by an application in the Mac itself.

There is no difference whatsoever in the file type itself, merely that one is a hangover from DOS-compatibility days. Filesystems which expect to write to FAT32 volumes such as SD cards, still maintain that 8.3 upper-case naming convention, for compatibility.

The Mac itself is by default not case-sensitive, so the files ABC123.JPG & abc123.jpg would be considered identical names. You can enforce case-sensitivity, but there are few use-cases for it for most people.

Lower two files came straight from the camera, above that are two conversions made within Nikon's own software, which preserved the filename & added an upper case JPG, top file is from photoshop which prefers lower case for no particular reason I can think of.
Late edit Photoshop's predilection for lower-case is actually a pref in the app itself. Take your pick.

[Neither of these is relevant to the situation itself, but the underscore prefixing the file names is another naming convention which merely signifies the pictures are originally saved with an Adobe RGB 1998 colour profile, as opposed to sRGB, which would use the format DSD_5371.NEF instead. The dash after some of the names is another application convention to prevent duplicate names.]

DSC & IMG are also conventions & in themselves have no real significance at consumer-level. iPhones save IMG, cameras use DSC [DSD is a manual change, done when 10,000 images have been taken, again to prevent name-clashes]

This is just how the files were created.

Someone, or some piece of software, chose to use .JPG in some cases and .jpg in others.

That's it. It doesn't mean anything.

Mac's filenames are case sensitive.

Some FAT filesystem file names get converted to all uppercase; particularly on older devices.

Unix based systems tend not to rely only on the filename extension to determine their types.