Omission??

[Eddie88]

Most of the time, the image is larger than the original, although (the image/it is) sometimes not.
With adverbial clauses beginning with while and when, the verb to be and the subject can be omitted, often leaving an ing clause.

In the sentence above, however, we are dealing with a different construction: .

With regard to the sentence above, is there a rule accounting for the omission of the verb to be and the subject, or is it simply an ellipsis of words that are obvious and thus left out?

(I'd say it is simply an ellipsis, but I want to know for certain. Thanks)

[Marius Hancu]

The proper version is none of the above, but, IMO:

Most of the time, the image is larger than the original, although sometimes it is not/it isn't.

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Infancy and developmental psychobiology‎ - Page 66 (http://books.google.com/books?id=H_BGAAAAMAAJ&q=%22although+sometimes+it+is+not%22&dq=%22although+sometimes+it+is+not%22&lr=)

by Paul Henry Mussen, Joseph J. Campos, Marshall M. Haith - Psychology (http://books.google.com/books?q=+subject:%22Psychology%22&lr=) - 1983 - 1244 pages

This is usually so, although sometimes it is not (Hinde, 1974).
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[Eddie88]

Any thoughts regarding the omission, the way it is written in my sentence (without the words)?

[Marius Hancu]

My thoughts have been already expressed.

[Eddie88]

I was speaking to everyone, not just you.

[Rusty]

With adverbial clauses beginning with while and when, the verb to be and the subject can be omitted, often leaving an ing clause.

In the sentence above, however, we are dealing with a different construction: .

With regard to the sentence above, is there a rule accounting for the omission of the verb to be and the subject, or is it simply an ellipsis of words that are obvious and thus left out?

(I'd say it is simply an ellipsis, but I want to know for certain. Thanks)

IMO it's a simple ellipsis. "Sometimes...sometimes not" is natural as in the following examples from Google Books.
In Shakespeare the vowel in such cases is sometimes slurred and sometimes not,
Sometimes inadvertently, sometimes intentionally, sometimes with the ... of the
author for aesthetic improvement or necessary correction, but sometimes not.
Fortresses are sometimes useful, then, and sometimes not;
Irregular relation between x and y means x is sometimes related with y but
sometimes not.

[Marius Hancu]

More feedback (from Mark Brader):
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1. Most of the time, the image is larger than the original, although sometimes not.

2. Most of the time, the image is larger than the original, although sometimes it is not.

Do both work, both formally and informally?

In #1, "although" feels unnatural to me, although I don't think I'd call it wrong. The usual word is "but". In #2, either word works.

In #2, "it is not" is only normal in formal usage. In informal language it contracts to either "it isn't" or "it's not", whichever the speaker prefers. (The uncontracted form is possible in a situation where there is special emphasis, but I can't see that occurring in this snetence.)

In both versions, the first comma is optional and, although I generally like optional commas to be used, I would prefer this one to be omitted.

Eric Walker writes:
I think so. But the question arises as to why, given that "most of the time", the words after "original" are needed at all.

For one thing, it allows for a possible distinction between "sometimes", "occasionally", "very rarely", and other such terms. It also provides a bit of emphasis on the "not" side.
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I'd still use only 2 in formal writing.