The problem isn’t in the cruise control. As you suggest, the power-to-weight ratio is lower on many small vehicles. This necessitates more throttle to negotiate hills, which in turn eventually makes the transmission downshift with a bang to try to maintain speed. Some manufacturers, specifically ones with a more expensive product than your vehicle, have engine controls with more intelligence: they are less prone to downshifting when in cruise-control mode than not, specifically to avoid this issue, which is referred to as hunting. On longer hills, the vehicle will go back and forth repeatedly between two gears as the transmission searches for the sweet spot.