Casual internet slang;
瓜 (melon) = juicy gossip, from
吃瓜群众 ('melon-eating bystanders').
大瓜 upgrades it to a major story (celebrity affair, corporate feud, viral drama). Common companion phrases:
蹲瓜 (wait for gossip),
瓜田 (the gossip field),
前排吃瓜 (front-row seat).