That's not really an answer to my question. Why is it so heavy? It's heavier than all of our Type-209s too. Are they also poorly designed?
The answer to your question:
TN has requested additional equipment and requirements from the sub and it has been re-designed with Turkish engineers, that required size/displacement to increase.
Meanwhile the increase in the size has resolved the stability issue that caused ship to have a dead-list under certain conditions which was earlier experienced by Greeks and Koreans, for the Greece a solution was found by Germans, Koreans introduced their own solution, Turks have known this issue and applied their solution on the design, by proving a space for the requirements.
It wasn't only the stability issue to be overseen, Turkish engineers also commonized the connections elements, hull support members to ease maintenance, a difference which has not been applied by koreans.
Meanwhile they did also detect the source of a noise which Greece and Korea had earlier been complaining of, and resolved it. A design modification was suggested by them and later applied by Germans.
Moreover it wasn't just bolt and nuts that turkish engineers have been through, they have overseen the design in details to suit it to Turkish Navy's requirements. The same stuff was also done for 209s in the past.
It is not 'heavier', it is bigger, specifically longer, thus it has more displacement. Comparison with Type 209 is pointless because 209 was already a matured design.