Grease. The more slippery you are the easier
Other than that aggresively go for the take down in the first place. I am myself pretty big, and it isn't easy for others to get me down (RAWRRR), but an aggressive takedown low on my legs (ankles) is what people do to me a lot.
You said the guy is 'heavy' but I'm guessing you are saying fat rather than muscular - in which case try to grab an arm and use a submission