i mean grapefruit size is big..not many fruits that i can remember are this big and purple.
but let's try:
purple passion fruit, caimito, plum,prune, davidson's plum, purple dragon fruit, mangosteen, purple custard apple, if you count eggplant or tomato, purple figs not big enough. can't think of anymore. A syzygium(more close to pink than purple but maybe)
internet and unlikely: carissa carandas, purple tomatillo, akebi(japanese and not round),Dacryodes(not round),Palmyria (nungu)
well if i had to bet my life it's either caimito,mangosteen or a plum. what else can be this big and purple?