iPhone 6S Plus 64 GB phone's backup size is 220 GB

My iPhone 6S Plus has storage of 64 GB, but the backup size is 220 GB. I am taking the backup on a Windows desktop computer using iTunes.

I have the 15.6.1 update installed on my iPhone, and my iTunes version is 12.12.4.1 (both the latest). I have the latest version of Windows 10.

I have been having this issue for maybe 6 months now. I contacted Apple phone support 3-4 months back (twice - two different people on two different days the second one being a supposed senior), and they took online access to my system and looked a lot, but could not debug the issue. They checked that all softwares (my phone, iTunes and Windows) were up to date. They asked me to try installing iTunes on a completely different system and backing up there, and I did it and the exact same issue happened again.

They then appeared to shift the blame onto Windows saying it must be a Windows issue, and left things there. If Apple suspects it is a Windows issue, is it Apple's responsibility to investigate it and then work with Windows to get it fixed, or mine? I really do not have the resources or knowledge for that kind of investigation - the Apple support team should have investigated the issue until they fixed it, not left it in between with some kind of guess. I think all of us buy iPhones under the basic assumption that it will get backed up by iTunes in a Windows environment.

Needless to say, this is very concerning. If I ever need to restore the backup to my phone, it will not happen since a 220 GB backup cannot be restored onto a 64 GB phone. Also, if I buy a new iPhone, I will need to buy at least a 512 GB phone since the restoration of this backup onto the new phone will take 220 GB. And I will forever lose some 160 GB of storage for nothing.

I request anyone with any tips to kindly let me know. Thank you so much for your time!