Not a biologist, but the evolution of mammals is way more granular than you might expect. Humans are the sole surviving species of the genus Homo, which was a real party before the other ones went extinct. You're in for a fun time.
Domestic cats are believed to be domesticated not from tigers, but from the African wildcat:
Which evolved to be small just because it's sometimes more useful to be small.
And no, hamsters are not off-brand rats. They're part of the rodent order, which includes beavers, moles, capybaras, guinea pigs (yes, also wild) and lots of other fun things: