Again, it is not the quantity,or even illegal content of the collection (although it makes it a no brainer), but the overall arching thematic nature of the collection. Also, they do not just collect photoghraphs, they scrapbook them, alter them, write detailed fantasies, or descriptions of actual events around them. Often a particular prideliction or fetish is obvious. It is unmistakeable and unmistakebly sick, but not necessarily illegal, under the child pornagraphy laws.
The average heterosexual adult doesn't keep scrapbooks of non sexual children's images culled from various media arranged/designed/restyled into erotica. Pedophiles do.
We don't know the nature of Westerfield's collection, only that "less than 100 questionable images" factoid. Meaning less that 100 might fall into the illegal pornographic definition. That's bad for the defense, but what's in the "legal" portion could be even worse if it is indicative of an obsession or fetish with children.
OTH, the rest may very will be adult pornography or landscapes for that matter, in which case the prosecution will have a difficult time proving its case for a sexual motive. IMO