Vuforia has a memory leak on iOS when the setting for the Target Resolution is not set to native.
For a while there has been an issue with the screen not filling properly under these circumstances, which could be fixed by editing the XCode project - however the memory leak is a complete killer.
You can verify this by using the FrameMarker sample and setting the Target Resolution to auto, best performance. Running with the Memory Monitor instrument will reveal a constant and fast increase in the Real Memory used by the application. Returning the setting to Native will remove this leak.