I noticed that in many Sifrei Torah, Sofrim prefer to "stretch" certain letters. It seems that these letters are stretched to end a line if there wouldn't be enough space to start a new word. That's not always the case, though; sometimes these letters appear in the middle of a row. Some sofrim use this technique often, while others don't use this at all.
Is there a reason for stretching the letter other than what I just mentioned? Is it some "artistic" form? Are there limits to either which letters are allowed to be stretched and how much stretching is allowed?
Note: I am not asking about large (rabbati) or small (ze'ira) letters. This has been addressed in a few other questions. I am talking about the horizontal size.