Arbitrary instance for generating unbiased graphs for quickcheck

module Main where

import Test.QuickCheck
import Data.Set as Set    

data Edge v = Edge {source :: v, target :: v}
                  deriving (Show,Eq,Ord)

data Graph v = Graph {nodes :: Set v, edges :: Set (Edge v)}
               deriving Show

instance Arbitrary v => Int-> Arbitrary (Edge v) where
    arbitrary = sized aux 
        where aux n = do s <- arbitrary
                         t <- arbitrary `suchThat` (/= s)
                         return $ Edge {source = s, target = t}


instance (Ord v, Arbitrary v) => Arbitrary (Graph v) where
    arbitrary = aux `suchThat` isValid
        where aux = do ns <- arbitrary 
                       es <- arbitrary 
                       return $ Graph {nodes = fromList ns, edges = fromList es}

This current definition of the instance is generating graphs with few edges, how do I alter it so it's less biased and it satisfies these two functions? :

-- | The function 'isDAG' tests if the graph is acyclic.

isDAG :: Ord v => Graph v -> Bool
isDAG g = isValid g && all nocycle (nodes g)
    where nocycle v = all (\a -> v `notMember` reachable g a) $ Set.map target (adj g v)

-- | The function 'isForest' tests if a valid DAG is a florest (a set of trees), in other words, -- if every edge has a maximum of one adjacent.

isForest :: Ord v => DAG v -> Bool
isForest g = isDAG g && all (\v -> length (adj g v) <= 1) (nodes g)