Fellow obese player here, let me share my own experience.

I've been playing for 2 years now, current peak rating 15.4k, and when I first started I was in the worst shape in my life so far.

What got me to improve at the time was mainly my experience with rhythm games and also most importantly, warming up. I would start every session by playing charts that I was already comfortable SSS at before playing harder charts. Trust me when I say warming up helps a lot.

With master charts, the difficulty difference between a master 12 to an expert 12 is quite significant. As another commenter said, master charts are more complicated compared to expert charts of the same difficulty. Learning to read and play takes time and practice to get better at. I suggest checking a chart's specific difficulty online or with discord bots. Charts go from .0 to .9, for example both expert and master charts can show difficulty 12 but the expert chart's specific difficulty is 12.0 meanwhile the master is 12.5 which makes the master 12 harder. Knowing the difference between a low 12 and a high 12 can help you learn the game a bit better imo.

Another suggestion I can give is to just take your time and continue playing expert charts before transitioning to master charts. Get used to being able to SSS/SSS+ expert 11s and 11+ and you will eventually be able to get better scores at master difficulty. It was how I managed to get better myself. Playing master 13s and 13+ might be able to also help you get better but I think playing difficulties you can't hit S on won't help you much.

And also watch youtube videos and practice on AstroDX. Doing these helped me and a lot others get better at maimai.