what does "-is saying something" mean?
".....In fact, I think he hates kids even more than dogs, which is saying something."

What does it mean? Does it mean "there are specific reasons to hate more than I know"

or "The reason he hates is not usual thing" or something else?

Question

what does "-is saying something" mean?
".....In fact, I think he hates kids even more than dogs, which is saying something."

What does it mean? Does it mean "there are specific reasons to hate more than I know"

or "The reason he hates is not usual thing" or something else?

Answer

It means he already hates dogs a lot, which means the hate he has for kids has to be particularly high.
Example:
He's more sporty than his sister, which is saying something.

The sentence implies that his sister is already very sporty, so him being more sporty than his sister is a particularly noteworthy achievement.

You could also say:
He's better than his sister at Math, but that's not really saying much.

This indicates that his sister is bad at Math, so him being better than her is not "saying much" - in other words, not a particularly impressive achievement, as most people are also better than her.

Answer

It means something else - it's a comparison. So in your example "he hates kids even more than he hates dogs, which is saying something" the person saying that sentence knows that the man really hates dogs, so for him to hate kids more than that is quite significant.

To give another example, if I said about myself "I love bacon even more than I love chicken, and that's saying something." I'm telling you that I like chicken a lot, but that I like bacon even more than that, so my love for bacon is very significant.

You generally use it with a friend, or someone who is familiar with the subject you're talking about. In my example I wouldn't say that to someone I didn't know; I'd say it someone who already knew how much I liked chicken, so that when I tell them I like bacon even more than that they would understand the significance of it.

Answer

It means he already hates dogs a lot, which means the hate he has for kids has to be particularly high.
Example:
He's more sporty than his sister, which is saying something.

The sentence implies that his sister is already very sporty, so him being more sporty than his sister is a particularly noteworthy achievement.

You could also say:
He's better than his sister at Math, but that's not really saying much.

This indicates that his sister is bad at Math, so him being better than her is not "saying much" - in other words, not a particularly impressive achievement, as most people are also better than her.