Just like Pat stated, it's crucial to to bring your legs to your chest using your entire core. This will help take their base away. Against larger opponents, trying to lift up on the hips at the same time normally isn't effective. I use my underhook and drive it in an explosive fashion in the direction I want my opponent to move at the same time I'm bringing my legs to my chest. I've rolled against some big dudes in my time, and this normally creates enough space to get to my hip and bury my head just below the sternum, which is where you want to be. Hope this helps.