Rihanna to Chris Brown -- How Could You Leave My Ass?

Rihanna just gave ex-boyfriend Chris Brown a little taste of what he's missing ... her perfectly sculpted ass. RiRi just posted an extreme close-up shot of her nearly bare derriere in shorts that are so tiny, her THONG is hanging out of 'em. Rihanna added a caption saying the pic was from the set of her new music video ... but we think it's pretty obvious the photo was for an audience of 1 -- Chris ... who was recently spotted out with his other sometimes-GF Karrueche Tran (who definitely doesn't have an ass like Rih). 

Think he's regretting the breakup yet? 

Wow nice ass rih rih bur seriously they look like remoulded clay to me, no wonder chris left. No beef sis bur who wants to have kinda thing in his home? #StoneAss