Symbolism, Imagery, Allegory
Q: What's big, heavy, and chained up?
A: Victorian female sexuality! (And also, the narrator of "The Yellow Wallpaper"s bed.)
We know we just made Victorian women sound super-kinky, but we're actually just alluding to the fact that female sexuality in the Victorian era was an all-consuming subject but also completely repressed. (Some people—lookin' at you, Freud—say that it was so all-consuming precisely because it was so repressed.)
The Victorians liked their women meek, motherly, and totally uninterested in sex. But, um, since the women in the Victorian era were human, that wasn't exactly the case. Enter: systemic repression. Women were expected to be pure and virginal and, even after marriage, find sex distasteful. This cultural standard was just as cumbersome, and as confining, as the bed that the narrator in this story is forced to lie in.