What makes an argument stronger is strength of the connection between the premises and the conclusion.

In the tiger argument, it is explained that tigers (one example of mammals) are aggressive and can cause serious injury to people. Therefore, we can conclude that it is true that "not every mammal is suitable to keep as a pet.

In the trash bin argument, we aren't given any solid premise that connects Mr. Cat's post-eating customs to the contents of the trash bin being spilled/ Mr. Fat Cat being guilty of making this happen. For example, there isn't a connection between Mr. Fat Cat eating and anything having been eaten out of the trash.

I don't know if I made any sense, but I gave it a shot.