Cruise control is not generally considered to be a fuel economy feature. It's mainly intended for comfort and convenience on long highway trips, where fatigue can be a problem.
Traditionally, cruise control systems try to maintain a constant speed over time without driver intervention. They will accelerate to maintain that speed when going up a hill, for example. In a very hilly environment, cruise control will definitely increase fuel consumption compared to driver control.
You didn't specify an exact model, year, etc., and there can be big differences in how cruise control systems are implemented. Generally it involves monitoring the vehicle's speed, engine load, and other factors, and making automatic changes to throttle position, engine timing, etc. to keep the speed constant.
In recent years, the development of adaptive cruise control systems which can use sensors to measure the distance to other cars and automatically adjust speeds has made them more useful in shorter trips and start-and-stop environments, but these systems are still primarily designed for convenience, with an emphasis on safety, not fuel economy.
How cruise control will compare to manual driving also depends heavily on the driver. If you're the kind of person who finds yourself creeping up to higher and higher speeds, then having to slow down when you realize it, then cruise control could definitely help reduce fuel consumption, but that's more a case of eliminating bad driving habits than an actual improvement in efficiency.
Add into this the fact that hybrids have two different kinds of economy (fuel and battery) to be concerned about, and you can imagine that the problem of maintaining a constant speed under varying conditions can be a challenge.
There is research being done on adaptive cruise control and how it can be made more efficient, and in some cases could actually create significant fuel savings, but that's still new technology and it probably won't be mainstream for a few years yet.
A human driver can rely on memory, knowledge, and visible clues (signage, road conditions ahead, level of traffic) to make smart decisions about how to drive and when to speed up or slow down. Cruise control is much more limited, only able to use the information available from sensors (and in more recent cases information from internet databases.) One of the things that enables more efficient cruise control is getting advance information about upcoming conditions, predicting when power will be needed, and making most efficient use of gravity, etc.
Also keep in mind that cars are different, and cruise control systems are different. If the cruise control system was not designed specifically to improve fuel economy, it probably won't.
In your particular case, it's possible you are just a very efficient driver, and the cruise control can't do as good a job as you can. There could also be a problem with a sensor, leading your car to make incorrect decisions about when to accelerate. That could certainly cause more fuel use.
