“Sometimes you notice a bulging cell”
I had a couple of large auxiliary battery packs for my computer, iPad and phone when I’m away from an outlet. I noticed one of them was bulging a few years ago and immediately tossed it. The manufacturer sent me a replacement based on the photo I sent them.
Is there a capacity / size below which there is no problem? What about one-time, disposable Li-ion batteries? Do they ever catch on fire or explode?
This would be unlikely unless you were trying to charge batteries that were not designed to be recharged. It is like charging alkaline batteries that were not designed to be recharged... most types can take a few cycles, but they get hot when you are doing it and they can burst and they are very likely to start leaking a caustic solution that screws up the contacts in whatever you are using them in.
They actually do make some alkaline batteries that are designed to be recharged. But you need to use a special charger that has been designed to be able to charge rechargeable alkaline batteries.
https://www.amazon.com/Alkaline-Rechargeable-Controls-Electronic-Christmas/dp/B07C96TWHW/