personally, against bigger guys ive always been partial to going low once you get their back. try a single leg, although i tend to use it more as an ankle block. drop down with your hand reaching to his shin/ankle and your sameside leg forward inside his. if you drop your opposite shoulder into the small of his back and drive through, you dont hafta pull his leg up, just stop its forward progress.
and for what osiris recommended, after gaining position first break his base by putting your foot as close to the outside of his as possible, and sinking down driving the knee into the side/back of his knee. this helps move those stubborn giants if their weight is on the side you want to take them to. then continue with sweep as described.