Usually my fantasies are kind of like those male "cuckold" ones, but I imagine my boyfriend being seduced by a busty woman and her doing all the things I can't while I watch and she insults me. Or just anything really where men are ignoring other women and are completely fixated on one woman's huge breasts. Or just me, myself, being the one seduced/insulted for having such an inferior chest.
I've always wanted to see a porn scenario almost exactly like what you just described: Where a plain flat-chested girlfriend comes home and catches her boyfriend fucking some bimbo chick with GIGANTIC tits. The girlfriend is devastated and asks why, and the boyfriend admits that he's always been a secret big boob fetishist and he just couldn't deny himself the pleasure of big boobs anymore. He keeps on fucking the busty bimbo who insults the girlfriend by making fun of her small tits, saying she's not a real woman, doesn't have the equipment to truly satisfy her man, while the girlfriend sits there humiliated but still watching them. The busty bimbo would add insult to injury by using her huge tits to full effect on the boyfriend: shoving them in his mouth, smothering him in her cleavage, motorboating him, titfucking him, etc. And while doing this she's saying stuff to the girlfriend like "you see how much he loves my big tits?", "you could never give him what I'm giving him", "his dick gets soooo hard when I shove them in his face". The boyfriend is so tit-drunk he doesn't even respond to his girlfriend's pleas, he just grunts and moans and devours the *WOMAN*'s huge boobs while she laughs and eggs him on. The girlfriend has never seen him be so raw and animalistic during sex, further humiliating her because she knows she could never turn him on like that. And for the final insult, he pulls put and shoots the biggest load the girlfriend has ever seen, all over the busty *WOMAN*'s tits while the boyfriend grunts like a caveman. The girlfriend has never seen him shoot anywhere even close to that much cum with her, but the busty bimbo turns him on so much he blows a Peter North sized load.