It does violate the "rule" where like should be compared with like.
The fuel efficiency of the 2019 model is 20% better than the 2018 model.
The height of the architect's new skyscraper is fifteen storeys more than his last building.
Speakers frequently leave out that of. The phrase that of is in a rather formal register. The percentage of speakers who actually use it in conversation is quite small. To the extent that a huge percentage of speakers commit this "error", and yet their meaning is perfectly well understood, we should probably classify this as a stylistic goof, not as ungrammatical. It's natural but potentially unclear.
You're not likely to encounter this "violation" in sentences as simple as:
John's hat is bigger than Peter
because it's easy to say "Peter's".
John's hat is bigger than Peter's.
The behavior arises when using phrases like "the height of the new skyscraper" or "the fuel efficiency of the 2019 model" as grammatical subject; such noun phrases are rather more complicated than "John's hat". By the time the speaker reaches the complement of than, the speaker may have lost track of the grammatical subject if it is a phrase of that nature.