It’s not nonsense though... the main cause of pain from large breasts is not having a correctly fitting bra; women on bra/breast related subreddits and IGs will tell you as much.
Multiple surveys show 80%-90% of women are not wearing and do not know their right size; chances are if Victoria Secret has been telling you you’re a 36C it’s more likely you’re a 28F.
That’s why I always recommend UK sized brands as they understand the importance to health of having well fitted bra.
However, back pain and other health issues are unavoidable past certain sizes, Macromastia is defined as a total breast weight being over 9.6lbs (~32K), Gigantomastia is over 14lbs (~32LL), the latter of which I’m sure Missy is beyond. Still, even at these sizes having a bra that fits will mitigate a lot of the problems.
If you really want to keep an accurate track of what bra size you need I recommend (and request) you should use this calculator:
https://www.abrathatfits.org/calculator.php
From the Reddit page r/abrathatfits, and use UK sizing (the sizes are consistent between brands and go up to larger sizes than the US).
At large sizes you need to take more than just overbust measurement, you need a leaning and lying measurement as well.
I also would suggest keeping track of their weight. You can do this by standing on some scales, having a second person take the weight of your boobs, and noting the difference on the scales.
This is so helpful!
If you want some specific brands I’d recommend for your size I’d look up
Ewa Michalak (they go up to an N cup and do custom sizes),
Molke (go up to an N cup, have their own calculator and system, you’d probably need the size “LThunder”) and
Optifit (tailor makes bras specifically for your measurements).