They don't all have to be extremely heavy. DT needs to be disruptive. Some dudes are crazy productive and aren't big. BSU's star DT was only like 260. I have no idea how he was so good but it must have been the athleticism he could display at that size.
But I like the idea of getting really athletic guys and moving them up. Get a big athletic safety and make him a LB. Get a LB and make him a DE. A DE who can move inside like Saleapaga, Fauatea, and others.