Explain the use of "mouthful" in: I've always called myself "Henny" because it's less of a mouthful than "Henrietta."I think it should be one of...

First: You are sorting out
idiom usage. Understand that
figurative idioms also have
literal meanings. Literal meaning describes a thing as it
really is: e.g., "The wedding gift was a crystalline canopy" is a literal use of
"crystalline canopy," while "The sky was a crystalline canopy" is a figurative use of
the same phrase.

Mouthful
may be used literally or figuratively. You may say "I have a mouthful of
alfalfa spouts" to literally describe what you are eating or say "Asking for
oscillococcinum is a mouthful" to figuratively express the difficulty of saying that
word.

Second: You are sorting
out the language of comparison. In English there are
prescribed rules for the language of comparison. One example misuses the language of
comparison. The phrase "less than Henrietta of a mouthful" breaks the usage rule, which
is the pattern "Something is less than something of something." In particular, "it’s
less than Henrietta of a mouthful" incorrectly adds the article
a to the pattern with an incorrectly
used preposition of. This phrase is
unacceptable in English. 

Third:
You are sorting out use of the preposition of. When
of is used following a proper noun, it
is locative or possessive: it indicates a location or a possession. An example of
locative of use is the phrase "Macbeth
of Scotlad.” It means that Macbeth lives in the location Scotland. An example of
possessive of is the phrase "Chaucer
of writing fame." This indicates Chaucer as the possessor of writing
fame.

You define the meaning
of the figurative idiom "mouthful" correctly. Remember though, if used in the wrong
phrasing, it ceases to be a figurative idiom and takes on the literal meaning of how
much you have in your mouth: a mouthful. In, "I've always called myself "Henny" because
it's less of a mouthful than "Henrietta," the idiom "mouthful" is in
correct phrasing, which positions it as a figurative
idiom.

Your suggestion that
"this sentence is usually ordered like the following: “I've always called myself ‘Henny’
because it's less than a mouthful of ‘Henrietta,’” is an incorrect
suggestion. The phrase "is less than a mouthful" uses
mouthful in its literal meaning because it fits the pattern of
literal comparison: Something is less than something else: mouthful
is no longer a figurative idiom in this sentence. The sentence now means that ‘Henny’
has a quantitative measurement that is less than what a literal mouthful of a person
called Henrietta is. So the rearrangement of the phrasing from "less of a mouthful than"
to "less than a mouthful of" has rendered the sentence illogical and absurd. Certain
patterns of English words must be honored or the intended meaning is
lost.

Finally, "I've always
called myself 'Henny' because it's less than 'Henrietta,' which is a mouthful" is an
acceptable sentence using a nonrestrictive
which-clause to elaborate on the name, with
mouthful as an idiom. Similarly, "I've
always called myself 'Henny' because it's less than 'Henrietta' of a mouthful" is
unacceptable in English because (1)
mouthful is no longer in a correct idiomatic structure;  (2)  it
now expresses a literal, not figurative, meaning;  (3) mouthful is
neither a place following a locative preposition
of nor is it a possession following a
possessive preposition of. In summary,
when you alter the rule of how words or phrases are used and ordered, they take on
different meanings. In this case, mouthful stops begin a figurative
idiom and takes up a literal meaning.