Animals (specifically corvids) don't measure things by weight and determine an exchange rate, they just look at them with a 1:1 ratio and check for similarity.
In this particular set up, they would simply correlate food with particular types of trash that they have been trained with and dump only those kinds of trash into the apparatus. To them, tearing up the trash would be more effort than finding new trash.
A crow at the beginning of this talk figured out how to bend a metal wire to use as a tool when a straight wire wouldn't work.
https://www.youtube.com/watch?v=0fiAoqwsc9g&t=17s
And the crow in this video figures out how to drink water at the bottom of a bottle by displacing it with pebbles.
You assume that to corvids, tearing the trash into two is an easier task to experiment with. It's not. Corvids, just like smarter humans, look for the easiest way to do something. Like another one of the children comments to mine says, they would rather find another way to "reuse" the trash, perhaps by finding some opening to the trash storage, or finding a nearby huge stash of trash which was not the objective, such as a landfill, or experimenting with other materials to see if they work (the reason why mechanical contraptions would barely work with magpies, etc. but would work with most other birds).
Apparently, a dolphin Kelly discovered the exchange rate to be one fish per one piece of paper... thus tearing a larger piece of paper into smaller parts, and getting a reward for each small piece. They 'gamed' the exchange rate into their advantage.
> This behaviour is interesting because it shows that Kelly has a sense of the future and delays gratification. She has realised that a big piece of paper gets the same reward as a small piece and so delivers only small pieces to keep the extra food coming.
Once they figure out what the reward system is, they often try to figure out how it can be gamed.