Take with a grain of salt as I'm an armchair grappler (meaning I watch a ton of material, but unfortunately don't train). I'd recommend watching Marcelo at absolute. Granted he's not a rubber guard player, but he taps much bigger men. IMO, I don't think I'd want to play RG on a guy if you can't do the technique right because of their size. Otherwise, roll with bigger guys and get a feel for the adjustments you'll need to make. But again, take what I say with a grain of salt.