I think that with 1/8 components a 1/5 size vehicle can be made that stays well under 20lbs.
As I was building mine, I thought it would weigh in at 17-18 lbs. But with Baja wheels and tires and 8s 5000mah batteries, it runs at 18.9 lbs now. Under 20 lbs, yes, but it would be difficult to get it a lot lighter.