The maximise button works differently in OSX to how it does in windows. It makes the window as large as it needs to be to view the contents without scrolling (if possible) and not a single bit bigger. There are some hacks out there to make it behave in a windowsy way, but I didn't bother and got used to the way it works.
You can always resize the window yourself by clicking on the bottom right corner and adjust the size that way.
If you want to cover the dock (the scrolly icons along the bottom), you can set it to auto hide like the task bar in windows. There's a tick box somewhere in Applications > Settings > Dock.