Why are the alias so big in filesize in Mountain Lion?
Since Lion, in addition to storing Retina icons, it seems that an alias stores its many different sizes of icons in both the data and the resource fork (the xattr com.apple.ResourceFork). Possibly in a move away from Resource forks, while still supporting previous OSes (for now).

The alias I just created contained the follow formats:
is32, s8mk, ic11, il32, l8mk, ic12, ic07, ic13, ic08, ic14, ic09, ic10

They are described here: http://en.wikipedia.org/wiki/Apple_Icon_Image_format