AllSync & AllDup

I don't grok the following. Is it flawed?

Compare Size

Here you can specify the maximum width and height of the pictures to be compared. A lower compare size finds more similar pictures and speeds up the comparison time. A higher compare size finds more identical pictures and less similar pictures and of course needs more time to compare them.

1. How can a higher compare size find "more identical pictures and less similar pictures"? The set of identical pictures ⊆ set of similar pictures. Thus the more identical pictures you find, the more similar pictures you find!

2. Why does a lower Comparison Size — e.g. 8 × 8 — find more similar pictures, and need LESS time?

3. Why does a higher Comparison Size — e.g. 24 × 24 — need MORE time?

Please explain like I'm 5. English ISN'T my first language. I'm computer challenged.

1.: The more data there is to compare, the more accurate the comparison. More accurate = more identical pictures.

2./3.: Why does a 1 MB file copies faster than a 1 GB file?