When I type a space after a word, the cursor moves to the right by the width
of a fairly large space. Then when I type the first letter of the next word,
the size of the space shrinks down to a more normal width. Why does this
happen, and why only in some documents?

Also, I find that the gap that Word gives me if I type one space after a
period at the end of a sentence, is too small, but if I type two spaces Word
gives me a gap that's about 5 times bigger than the gap with one space.
What's going on?