Wrong scaling in xyz coordinate systems

Wrong scaling in xyz coordinate systems

As stated in the Manual in section 10.2.1 "the default z-vector points to
(-sqrt(2), -sqrt(2)). But when I draw:
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[axis/.style={thick,->}]
\coordinate (O) at (0, 0, 0);
\draw[axis] (O) -- +(1, 0, 0) node [right] {$X$};
\draw[axis] (O) -- +(0, 1, 0) node [right] {$Y$};
\draw[axis] (O) -- +(0, 0, 1) node [above] {$Z$};
\draw[red, thick] (O) -- +(-0.70710678, -0.70710678);
\end{tikzpicture}
\end{document}
I get this:I cannot post an image, as I don't have enough rep...
As you can see: the red line is clearily longer than the z-axis, whereas
it should be equal in length. Can someone point me to the right direction?
During my investigations I found another strange behaviour:
\documentclass[11pt]{minimal}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\draw[red] (-2cm,-2cm) grid (3,2cm);
\coordinate (O) at (0, 0, 0);
\draw[blue] (O) -- +(0, 1cm, 0);
\end{tikzpicture}
\end{document}
Yields two pages: the first one is empty, and the second one Looks like: I
cannot post an image, as I don't have enough rep...
This is the whole page, I didn't clip anything.
I am totally confused.