Why is the inside margin so narrow?

If you give the standard classes the twoside option, the class sets the margins narrow on the left of odd-numbered pages, and on the right of even-numbered pages. This is often thought to look odd, but it is quite right.

The idea is that the typographic urge for symmetry should also apply to margins: if you lay an even numbered page to the left of an odd-numbered one, you will see that you’ve three equal chunks of un-printed paper: the left margin of the even page, the right margin of the odd page, and the two abutting margins together.

This is all very fine in the abstract, but in practical book(let) production it only works “sometimes”.

If your booklet is produced on double-width paper and stapled, the effect will be good; if your book(let) is produced using a so-called “perfect” binding, the effect will again be good.

However, almost any “quality” book-binder will need some of your paper to grab hold of, and a book bound in such a way won’t exhibit the treasured symmetry unless you’ve done something about the margin settings.

The packages recommended in “setting up margins” mostly have provision for a “binding offset” or a “binding correction” — search for “binding” in the manuals (vmargin doesn’t help, here).

If you’re doing the job by hand (see manual margin setup), the trick is to calculate your page and margin dimensions as normal, and then:

which can be achieved by:
(substituting something sensible like “5mm” for “offset”, above).

The above may not be the best you can do: you may well choose to change the \textwidth in the presence of the binding offset; but the changes do work for constant \textwidth.