There probably is a huge amount of disgusting child porn in the world.
But, the bias and culture reflected in the laws, may increase the "raw number" in surprising ways. E.g. in Denmark porn was made legal in 1969. Any porn -- all restrictions was lifted. This means that sexually explicit photos involving children was legal for a (small) number of years, then the law was changed to limit legality to "above 15 years of age". For a long number of years, it was legal to create porn, soft porn, and sexually suggestive images involving persons above 15 years. Then, I think about 2010, or so, the law was changed, such that persons now needed to be above 18 years of age.
There are tons of images of girls age 17 in sexually suggestive positions, etc, published between 197x and 2010, e.g. in tabloids, etc. These are now technically illegal, even though nobody batted an eye, when they were published.
Culture may also change. In my childhood, it was perfectly normal for children to be naked on the beach, and perfectly normal to snap photos of your children on the beach. You might easily get a naked child in the background, etc. Depending on context, such photos may be illegal now, and count in the statistics. The law is vague, and the police (for obvious reasons) may count stuff that is borderline, as child porn.
Disclaimer: I am no expert in this area, so years and such in the above may be of. I am just trying to argue that the number may be increased in surprising ways, that are consistent with the laws in a given jurisdiction.