Did they all come from the same source? If so I suggest you ditch that source! I think I once had a battery, probably from a random eBay seller, which was swollen. That suggests a fault in manufacturing or storage can be a cause, on the other hand maybe it was a battery recovered from a scrap phone.
If the batteries were from different sources logic suggests it has to be the phone. I assume the swelling only occurred after a period of use? Batteryuniversity.com is the fount of all knowledge on batteries but I don't instantly see anything there about batteries swelling. Maybe the charger is charging it too fast? Check whether it gets warm on charging.
I'm not sure whether the battery protection circuity is in the battery or on the motherboard. Check the battery voltage immediately after a full charge, or if you can, while still charging once it gets above 95%. It should be no higher than 4.2V. And what is it when the battery is allowed to discharge until the phone shuts down? The protection circuitry should shut t off at between 2.2 and 2.9V.