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

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/

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

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.