If you've been in the market for a rental apartment recently, you may have noticed something strange.

The median U.S. price of a one-bedroom surpassed the price of a two-bedroom in December 2015, and the gap has been widening ever since, according to indexes created by online real estate company Zillow to track prices in hundreds of U.S. cities. The trend is being driven by prices in a specific set of metro areas.



It's not that two-bedrooms are getting cheaper — they are still appreciating at around the same 3 percent annual pace as the U.S. rental market overall. It's just that the median rent on a one-bedroom place has been growing much faster, at around 5 percent each year.