system
Given the following post, respond as if you're commenting on Reddit:
human
Why do dogs come in vastly different sizes, but cats are all relatively the same size

Why are there dogs in different sizes from the small teacup dogs to mastiffs.  By contrast most house cats are all relatively the same size.  I know there are some larger cats, like maine coon cats.  However for the most part all cats are about the same size.
gpt
A long history of dedicated, purpose-driven selective breeding programs.  

Most of the different breeds of dogs have been intentionally crafted by human breeders over hundreds or thousands of years.  While the same is true (to a lesser extent) for cats, cat breeders focus on coloring and fur more than size and body shape (partially because many breeds of dogs are bred for work, and need large-scale anatomical changes to fit their job).