How do they match the weight of a prosthesis to the weight of the remaining breast?

I go to my local Myers where they have a lady who specializes in prosthesis fitting.You’ll probably find they go by your bra size and bring out various sized ones for you to try on. And trust me,you’ll know which prosthesis feels right/comfortable. You can always take it back if it’s not. Finding a comfortable bra is the harder part and I keep coming back to the Berlei post surgical bra.For me,it’s the most comfortable. It’s trial and error with bras