If you are using a graphical version of Emacs, you sometimes need to make things bigger or smaller depending on what you want to see or show to someone at your desk.
There are two ways to to this. The picture "small" is our baseline of a text file.
You can increase the text size with
C-x C-= or A-x text-scale-increase
which will zoom the content of the main window but leave your status bar the same as in the picture "zoom" below.
For a quick zoom in or out this works really well. But if what you really want to do is adjust your Emacs window uniformly you have to choose "Options -> Set Default Font" which in the Mac version pops up a font GUI that allows you to set the new font size for everything. Which uniformly changes the size of everything including the status bar.