question: what are the rules for occupying a creature’s space? I heard that smaller creatures can occupy a big creature’s space, but where is that? I can’t find it.
You can move through a nonhostile creature’s space. In contrast, you can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you. Remember that another creature’s space is difficult terrain for you. Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.
That said, there are creatures, mainly Swarms, which can occupy the same space as another creature. The caveat to this is that the ability to occupy another creature's space is indicated by their stat-block due to an ability or special characteristic.
question: what are the rules for occupying a creature’s space? I heard that smaller creatures can occupy a big creature’s space, but where is that? I can’t find it.
Extended Signature! Yay! https://www.dndbeyond.com/forums/off-topic/adohands-kitchen/3153-extended-signature-thread?page=2#c21
Haven’t used this account in forever. Still a big fan of crawling claws.
Page 191 PHB (emphasis mine):
That said, there are creatures, mainly Swarms, which can occupy the same space as another creature. The caveat to this is that the ability to occupy another creature's space is indicated by their stat-block due to an ability or special characteristic.