According to me, Notepad doesn't remember the last open Window behavior. If
you open a file in Notepad and set Notepad to maximize and the next time you
open the same file, Notepad revert to its original size.
You can do two things. Right-click on Notepad and choose Properties. From
the Shortcut tab and under Run set the value Maximized. So next time you open
Notepad for a new file, it will open Maximized.
Double-click on a TXT file to open it Notepad and resize the Notepad Window
to maximum you can. Next time it will remember that size but not the maximize
setting.
Hope this help, let us know!