You will see some of that with Linux, but not nearly as drastic. Software, by its nature, never gets smaller. (Sure, there are exceptions.) Then, you'll be installing your own software and making your own system changes - atop those made by the folks providing your updates.
For a new user, I recommend not tweaking it a whole lot, and sticking as much as they can with applications directly available in their repositories. Linux is remarkably stable until you go screwing with it. It's when you go tweaking and changing that it tends to fail.
Which is okay, if that's your route to learning. I call it 'learning by breaking' and it's a perfectly valid way to familiarize yourself with Linux - perhaps more so than by just generally using it and reading a few articles. Learning by breaking teaches you how the underlying system works, 'cause you've gotta fix what you broke. It's also generally a useful tool for teaching people how to properly backup their computer.
Anyhow, it'll slow down some. Not a whole lot, depending on what you do, but unlike entropy, software always moves to a more complex state.
