Software bloat is where you keep adding features to a piece of software, often beyond the point where they are actually useful. Think about what an application like MS Word can do nowadays compared to what it used to be able to do - yet many of the new features are used by very few people.
"Pressure of popularity" I would take to mean that software bloat often happens because of commercial or similar pressures to keep adding features because your competitors are doing the same, and people think they want lots of features.
So the law is intended to be a humorous expression of that. If you keep adding features to your program, eventually someone says "why don't we make it so you can read your email using it?"