While the fonts themselves are proportional, usually each letter (counting space as a letter) has the same width every time, it just happens that an 'i' takes up less space than 'w'. Regular spaces are usually 'en-spaces', meaning the width of the 'n' in whatever typeface you're using.

That being said, spacing is obviously very different in justified type (and a huge pain to set by hand), where it will change depending on which words fit in the line.