can anyone tell me how can i get more than 8.4 on my model score

8.4 is excellent score, you have nothing to worry about. Anything above 7 is good and you only have to try to improve it if you want to achieve perfection, which is pointless exercise in this business if you'd ask me.

Does the file size also affect the rating or just the quality?

Scoring systems can be tricky sometimes; improving technical details doesn’t always boost visibility right away. Rankings depend on more than the score, like competition and recent uploads. It’s a bit like custom mini cereal boxes: design quality matters, but placement, timing, and presentation also influence how much attention they attract.

https://www.packagingunit.com/custom-cereal-boxes/