They are different because Winword.exe is only the core executable. It "calls" many other files to assemble the final working program which might include many different DLL files to join with Winword.exe to make the program do what it does.
When you go to use various features of Word, different modules get loaded to do the task you are using. The process size is also looking at variable space Winword has allocated for it's use. If you load a 30 page document you'll see Winword grows quite large as a process.