Gosh @Oski - Personally, I haven't heard of this sort of extreme shrinkage - but I know that my own lumpectomy (with fat redistributed into the 'hole') has 'settled' over the last 5 years with a bit of a 'dip' - but not enough for me to worry about it, or need for further surgery.
Radiation can cause some shrinkage (and/or firmness of the tissue) ....
I've just searched 'breast shrinkage' and there are definitely a few saying the same ...
https://onlinenetwork.bcna.org.au/search?query=breast%20shrinkage&scope=site&source=communityHave you spoken with your GP about this? Is your surgeon still 'active'? Tho I guess you'd be seeing a Plastic Surgeon for reduction of the other breast ( .. just make sure they are proficient Breast Cancer surgeon!)
I would be surprised if it wasn't deemed medically necessary (both for your mental health and physical health) but who knows - they can be very good at getting out of not paying for stuff! ...
I guess it depends on how the PS (plastic surgeon) 'words it' and which category they choose for the 'forms' .... Definitely raise it with all your health team .... and I wish you luck
take care