JPG and JPEG are the same format
Both extensions describe the format defined by the JPEG standard. A file named photo.jpg and a file named photo.jpeg contain the same kind of image data, and software treats them identically.
Nothing changes when you rename an extension. The bytes inside the file stay the same, so renaming only affects which program opens the file by default.
Why the three-letter name exists
Early personal computer systems limited file extensions to three characters, so .jpg became the working shorthand for a JPEG file. The convention stuck after that restriction disappeared, and both spellings remain common today.
Search results, camera settings, and upload forms use the two spellings interchangeably. If a site only lists one of them, the other file still matches the same format.
Converting and shrinking JPG files
Because JPG and JPEG are identical, conversion only matters when you are moving to a different format. A JPG can be converted to PNG when you need transparency preserved from an original PNG, or to WebP when a smaller modern file suits your workflow.
To reduce the size of a JPG, use compression rather than renaming. Filekind runs in the browser and finds a JPEG at or below the limit you set, with image limits of 25 MB, 16,000 pixels per side, and 48 megapixels. The same limits apply whether the file arrives as .jpg or .jpeg, since both are read as one format.