I mean, not to get too armchair puberty expert about it, but I had gone through puberty by 16 and have been the exact same size since then. And as icky as I feel commenting on these girls' bodies like this, Kihira looks pretty physically mature already. She certainly didn't look like she was landing those 3As by being skinny yesterday; rather, she earned them with fast rotation.
All that is to say, I wouldn't be surprised if she kept the 3A for a while, at least in terms of physical ability. Headspace is a different matter.
Where did you see Trusova with puberty starting? Do you mean this? https://www.youtube.com/watch?v=6v_rH5xP34EI think the same.
Btw, I have watched Trusova’s LP from this weekend and puberty has started for her (which is the only fair thing in life, we all go through this). She sure will have to make constant adjustments as her body changes. Only time will tell how it will turn out.