I have the exact same issue, not only is the battery bloated it is also highly inaccurate when in use. the battery can last a day, but it can also be fully consumed by early afternoon, when used equally on both days.
I presumed the bloated state makes it malfunction, so the bloated state is what needs to be fixed.
as to how it became bloated, i can’t say I started to notice sudden drops in battery life … as in, 15% left, then opening a link in a mail, start video playback for 4 seconds and phone powering off due to 0%.
then i noticed some issues with the back piece not being able to close well.
and thats when i started noticing the battery being thicker than it should be.
but can’t say which one was first.