Cool and interesting article @chloroform!
they were more likely to devour the non-cannibal form of the tiger salamander, sparing the cannibal one.
I wonder why this happened? Perhaps because it is smaller than the cannibal one?
the other spiderlings might want to come and feast upon someone's mothers
That might sound extreme for us "advanced" humans, but insect lives are so different, sometimes they eat their partners as in the case of spiders you mentioned, and this time, they might eat their mother... such is the dangerous life in the insect world. Good thing for us we don't need to do any of that in order to survive.
Size could be one of the factors but the experiment conducted by David Pfennig suggested that they recognise similarities between them with the others through their keen sense of smell. They would most likely to spare a tiger salamander who smell like a cannibal than the others who do not as they feel there is some sort of biological relationship which they interpreted as being one of their kin.