The word 'penis' is implied in a sentence of that structure.
Note that the word 'penis' in the above example is context specific; it isn't a catch-all for all such sentences, e.g. "My hat is the same size as Judy Dench's".
( Bob Todd whee, Mon 5 Nov 2007, 0:14, archived)

But I'm not witty enough to make a joke out of that.
:(
( Shegetz Chris Kamara's comedy moustache, Mon 5 Nov 2007, 0:17, archived)