ZZ plants, like most plants, have an increasing tendency to be unable to support their own weight as the light level under which they're grown diminishes. To correct, more light is the best remedy, but pruning the plant is also a viable option, and it promotes a fuller plant. You can also strengthen stems by running your hand over them every time you walk by, thus flexing the stems. This stimulates lignin production - lignin is what makes trees woody & strong. A fan on your plants is also helpful. I think someone else just mentioned that (fan) on another thread recently as well.

Al