The max size a jump tender can move is the size of the ship OR the max rating of the drive, whichever is smaller.
So in your case, with a tender that is 13,000 tons, the max size of the ships it can escort is 13,000. Having a 25k max jump drive on a 13k ton tender, still can only escort ships of a max 13k tons. So make the tender bigger. And man those are slow, nuclear pulse drives are painful.