This title is also misleading, though. By claiming “evolution isn’t linear” and then showing a massive dinosaur leading to a chicken, you’re suggesting the chicken is a downgrade (otherwise, what “linear” would even mean in this context?).
The chicken is, however, a massive upgrade - for the specific environment it lived in. Well, “proto-chickens”, let’s say. The actual domesticated chicken is the result of artificial selection.
The issue is precisely in your mix up of “linear progression” and implying “smaller” is somehow a counter argument to that. While it’s true evolution isn’t linear, being smaller is not a downgrade at all.