I am having trouble understanding your question. I don't know what a "medium" jpg is. When I save JPEG files, I give them a length and width, as well as a resolution. They are all exactly the same size.
As for use in a low light gym shooting sports, I give the nod to the D600 over the D800. Though both will be quite superior to the D300 in terms of noise performance. The D800 may well seem slow to you for frame rate, and the D600 may seem slow for focus performance. Then again, it may not. Rent or borrow one for a weekend and see.