[Charon77]

Well it no longer constrains the data in a fixed output length.

[dbdr]

Sure, but if you constrain to fixed output length, you will definitely have collisions (Pigeon Hole Principle). There's no way around that.

[hsbauauvhabzb]

padding with zeroes to a fixed length and prepending the original length would suffice, but you’d have to have a fixed length of double infinity to account for both the length information and the hash information, and the hash is less efficient than the original information.