system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why are there no 'big' dogs, only cats?

Howdy howdy, I was wondering if there were any reasons (biological, physiological, geographical, etc.) as to why there aren't any tiger/lion equivalents for dogs?

Flip side then too, of why there aren't any wolfpack equivalents for cats?

Lastly if there were any instances for either which have died out, and why?

EDIT: sorry for potentially being misleading, the question wasn't so much as to whether dogs can be large, but more the chicken-and-egg argument for small size and pack hunting vs large size and solitary. Especially given that the distinction in behaviour and size goes all the back to sabretooths and direwolves.

Cheers for all the answers so far though!
gpt
Because they are pack hunters, and therefore don't have an evolutionary benefit from increased size. Feline species usually hunt alone, and therefore have an advantage if they are giant sabletoothed monsters.  
You should also keep in mind that chihuahuas and basically every dog 'kind' you see we not evolved that way, we artificially bred them from wolves to be cute and purse-sized.