once I read some where (in FDD methodology proposed by together) that a methodology should not be more than 10 page!
I have used this over time about RUP
the biggest problem with RUP is it's size
although it has lots of good practices that let you develop software fast. (like what agiles claim)
but since the learning curve is so lengthy, most project managers and their development  team never learn it completely,
and they usually end up using it in wrong way.
the best thing about agile practices are they are usually short and easy to learn, so PM and developers, can learn them and
apply them in their project very fast.