PHB p. 191 states:
[Y]ou can move through a hostile creature's space only if the creature is at least two sizes larger or smaller than you.
Consider a tiny enemy, such as a crawling claw, MM p. 44, occupying a 5-foot space containing a doorway. Two PCs, a Small-sized gnome and a Medium-sized human, are adjacent to it. The human, on her turn, uses her movement to move through the doorway, even though the crawling claw is in that space. Even if the optional Overrun and Tumble rules (see DMG p. 272) are in play, she needn't use them, because she is two sizes larger than her enemy. She does, however, treat the crawling claw's space as difficult terrain.
Next it's the gnome's turn. The gnome isn't two sizes larger than the enemy, so he can't move through the doorway. Period.
Is that really correct?