Macbook Pro mid-2015 15" swollen battery
No, it is not “generally expected” that batteries will swell after a certain amount of time.

What you have is a defective battery - it happens.  It needs to be replaced. 

The swelling is caused by trapped off gasses from the chemical reaction not venting properly.  It could swell and not cause any issues or it could go into a condition known as thermal runaway where it will burst and catch fire igniting the off gasses in an uncontrolled manner.

This is not typical, but it does happen. Batteries, like everything else produced by humans, aren’t “faultless.”  Trying to pin down how common it is, is like trying to determine your odds of getting a “lemon” Toyota.  It’s rare, but it happens.

MacBooks from 2013 on started using batteries that were glued into place requiring you to replace the entire top case.