I am formatting some diagrams using arrows in xy-pic. The "standard" arrow head that xy-pic produces with the command \ar is quite different (much smaller, thinner) than that produced by using the command \to (outside of the xy-pic environment). I like the fatter arrowhead much better. How can I tell xy-pic that this is what I want??

The difference in output can be seen by compiling the following code:

\documentclass{article}
\usepackage{amsmath}
\usepackage[all]{xy}

\begin{document}

$X\to Y$

$\xymatrix{X\ar[r] & Y}$

\end{document}

-

Use \SelectTips{cm}{12} just after the \usepackage[all]{xy} for computer modern tips.
Or you could simply say \usepackage[all,cmtip]{xy} (thanks to Phil Hirschorn).
Equivalently, say \usepackage[all,cmtip]{xy} – Phil Hirschhorn Apr 12 '11 at 0:29