can be a few variations that result to this.
1) those china non-branded charger
2) USB cables are those cheap and thin one.. not certified usb type c cable.
till now, i have never had a single phone/tablet battery that is bloated.
as most of the time i use Anker/Ugreen/Baseus charger.
Suspect Xiaomi Quality issues (batch related, not systematic problem).
My Mi Max 1 also got battery swollen (about 2mm) problem after 2.5 years. Always using the original Xiaomi Charger and cable. That is the worst we get from a phone since I got my first phone in 1999. We got other smart phones from Apple, Oppo and Huawei and all are okay in terms of battery life span. Even the two old dead Samsung Galaxy S4 phones did not have swollen battery problem (but other problems, including lousy battery life).
Then my iPad 2 also got battery swollen problem but that is after 6.5 years of active use and half years of inactive use. Always use official Apple charger and original Apple Lightening Cable or Ugreen Lightening Cable. It swollen very badly (more than 50mm). Luckily it did not explode. Still that is very respectable product life.
I have thrown away the two devices. Supposedly I could change the battery for Mi Max 1 but I lose the confidence of the phone battery since quite a few users in the Mi Max Thread got the same problem after two years plus of usage. It was one of the early large screen phone with big battery and maybe the battery technology was not that good.
We got battery swollen problems with Nokia phones last time but again that is after quite a few years of active use.
That being said, in the P20 thread two users also reported battery swollen problem after around 1.5 years of usage for their P20 Pro, a premium phone.