Here is a nice game I played in the Austrian attack against the Pirc defense with White. Counting pawns as pieces here, there are 32 remaining pieces!
[FEN ""]
1. e4 {Best by test} d6 {The Pirc defense} 2. d4 {the best move} Nf6 {theory} 3. Nc3 {theory} g6 {theory} 4. f4 {the Austrian attack is on the board!} Bg7 {theory} 5. Nf3 {theory} Nc6 {The first rare move, but my opponent cannot get me out of book that easily!} 6. Be3 e6 {another rare move} 7. Be2 Ne7 8. h3 b6 {still following Tzermiadianos-Nikolaidis} 9. O-O {the first novelty} Kd7 {does my opponent want to troll me?} 10. Qe1 Kc6 11. Qg3 Neg8 12. f5 Nh6 13. e5 Bb7 14. Rad1 Qd7 15. Bg5 Rad8 16. d5+ Kc5 17. Bd3 Rhe8 18. Nd4 Nhg8 19. Nc6 a5 20. Ne7 Kd4 21. Qh2 a4 22. Nb1 a3 23. Rf3 Ba8 24. b4 Bb7 25. b5 Ba8 26. c4 Bb7 27. c5 Nh6 28. Be3# {a beautiful model mate}
You may potentially have noticed, that this game might not have been played in a serious tournament ;) This is because I did not only play White but Black as well, as you didn't specify in your question which game source you accept.
Therefore, I consider this a totally valid answer that cannot be beaten!
It took me some time to compose. In the solution process, I proved that each heaviest model mate needs 8 participating pieces (pawns are excluded), and requires a promotion (which in turn requires a capture, reducing the number of stones on the board).
Therefore, there does not exist an even more heavier/beautiful model mate (as measured by participating pieces) in the set of heaviest model mate games (as measured in the total number of pieces + pawns at the end of game).
Edit: In the proofs, I didn't consider any pin exemptions, but only what I got as a definition from Hauke's question.