If weight gain also involved loss of fitness overall or you have been riding less, that will certainly have an impact on your stability.
I see from your previous posts you’re an advanced beginner. That’s a tricky place because ones skills are still tentative and so much can upset them. New saddle, new horse, wrong saddle, different stirrup lengths, the wrong riding pants or boots… almost anything can set you back and unbalance you.
Is the question whether very rapid weight gain can affect your balance and coordination? It might.
Or is the question whether weight loss will improve your seat? For someone who has been at a certain weight long term, losing 20 lbs probably isn’t going to give them a better seat. But fitness training especially core stability might.