Sentences with comparisons - similar structure, but different treatment?

[Charlottean]

I'm currently reading a book on English grammar and it has 2 examples of what seem alike sentences to me, but the logic applied to treating each of them seems inconsistent to me. Could anybody help me understand why these 2 sentences are treated differently?

"French wines taste better than Australian wines DO"
"I grow tomatoes that are bigger than a baby's head"

Gramatically these 2 sentences are following the same pattern:
A noun + a verb + 'than' + comparative word + [whatever the correct name of the second part of the sentence].

However, in the first example we need to use a verb replacement (DO), but in the second example we don't. Explanation I found in this grammar books is a bit confusing - it says that in the first sentence "better" refers to "taste", but in the second sentence "bigger" refers to "tomatoes", not "are". Logically I can understand this explanation, but I cannot figure out a straight-forward rule how to unambiguously identify what part of the sentence comparison relates to.

I would really appreciate if anybody can help me with this. Many thanks in advance!

[Jamie (K)]

This can be complicated by the fact that in the first sentence you don't even need the word "do". You can say:

"French wines taste better than Australian wines."
(Not always true, by the way. I've had some nasty French wines and some exquisite Australian wines.)

Furthermore, you could insert "is" at the end of the second sentence:

"I grow tomatoes that are bigger than a baby's head is."

I don't know that there's really a rule.

[Yankee]

Hi Charlottean

Basically, the primary difference between the sentences boils down to the fact that we don't replace the verb 'be' with the verb 'do'. I agree with Jamie that the word 'is' could be added to your second sentence.

In your first sentence, the word 'do' replaces the verb 'taste' instead of repeating it. Whether or not you replace or repeat the verb is optional here:
- "French wines taste better than Australian wines (taste)."
Breaking this sentence into two sentences works easily:
- Australian wines taste good. French wines taste better.
How Australian wine tastes is compared to how French wine tastes.

In your second sentence, the verb 'be' is not repeated. However, as in the first sentence, whether or not you repeat the verb is optional:
- "I grow tomatoes that are bigger than a baby's head (is)."
This sentence can also be broken into two sentences, but not quite as neatly:
- A baby's head is big (i.e. a certain "bigness" or size). The tomatoes that I grow are bigger.
How big a baby's head is is compared to how big the tomatoes are.
In other words, head is compared to tomato in terms of size/"bigness".
.