Two interpretations of “ if not more” in this sentence

Question

I’m Japanese and learning English.
I have a question about the meaning of “if not more” in the below sentence.

Many people who follow this diet end up eating just as many calories, if not more , because they constantly feel hungry.

According to my text, this means “... end up eating just as many calories as before they begin it, or even more calories than before they begin it.”

However, I interpreted as “... end up eating just as many calories as before they begin it, not to say more calories than before they begin it.”

Which of the two is correct? And why?

Answer 1

The placement of if not more in the sentence might be a little misleading. It actually goes with just as many, but it can be placed before or after calories.

...just as many, if not more, calories.

They eat just as many calories [as they would have eaten while not on this diet] and perhaps even more calories [than they would have eaten while not on this diet], because the diet makes them feel hungry.

Answer 2

Literally, than X, if not more is a short way of saying or possibly even more than that same X.

In your example the "than X" is understood.

If not more is used if someone's expectations are significantly below what happens in reality.

Many people who follow this diet end up eating just as many calories [than someone], if not more than that same someone, because they constantly feel hungry.