diff --git a/filter.go b/filter.go
index 271f050..5fb8a4a 100644
--- a/filter.go
+++ b/filter.go
@@ -149,7 +149,7 @@ func winnowNodes(sel *Selection, nodes []*html.Node, keep bool) []*html.Node {
 		})
 	}
 
-	set := make(map[*html.Node]bool)
+	set := make(map[*html.Node]bool, len(nodes))
 	for _, n := range nodes {
 		set[n] = true
 	}
diff --git a/traversal.go b/traversal.go
index 46bac05..eb88afb 100644
--- a/traversal.go
+++ b/traversal.go
@@ -149,7 +149,7 @@ func (s *Selection) ClosestMatcher(m Matcher) *Selection {
 // ClosestNodes gets the first element that matches one of the nodes by testing the
 // element itself and traversing up through its ancestors in the DOM tree.
 func (s *Selection) ClosestNodes(nodes ...*html.Node) *Selection {
-	set := make(map[*html.Node]bool)
+	set := make(map[*html.Node]bool, len(nodes))
 	for _, n := range nodes {
 		set[n] = true
 	}
@@ -689,7 +689,7 @@ func getParentNodes(nodes []*html.Node) []*html.Node {
 // Returns an array of nodes mapped by calling the callback function once for
 // each node in the source nodes.
 func mapNodes(nodes []*html.Node, f func(int, *html.Node) []*html.Node) (result []*html.Node) {
-	set := make(map[*html.Node]bool)
+	set := make(map[*html.Node]bool, len(nodes))
 	for i, n := range nodes {
 		if vals := f(i, n); len(vals) > 0 {
 			result = appendWithoutDuplicates(result, vals, set)