The Japanese I-400-class submarines/submersible aircraft carriers, which I believe are still the largest diesel submarines built, were under 7000 tons.
> which I believe are still the largest diesel submarines built
According to the internet you seem to be right, the Qing-class gets very close but not quite there: wiki lists 6628t, versus 6670t for the for the I-400.
That's quite massive.
The Japanese I-400-class submarines/submersible aircraft carriers, which I believe are still the largest diesel submarines built, were under 7000 tons.