diff --git a/pkg/dataobj/internal/sections/streams/streams.go b/pkg/dataobj/internal/sections/streams/streams.go
index a90c64cac1..3fc583342b 100644
--- a/pkg/dataobj/internal/sections/streams/streams.go
+++ b/pkg/dataobj/internal/sections/streams/streams.go
@@ -39,6 +39,10 @@ type Streams struct {
 	lastID   atomic.Int64
 	lookup   map[uint64][]*Stream
 
+	// Size of all label values across all streams; used for
+	// [Streams.EstimatedSize]. Resets on [Streams.Reset].
+	currentLabelsSize int
+
 	globalMinTimestamp time.Time // Minimum timestamp across all streams, used for metrics.
 	globalMaxTimestamp time.Time // Maximum timestamp across all streams, used for metrics.
 
@@ -112,20 +116,13 @@ func (s *Streams) EstimatedSize() int {
 		rowDeltaSize       = streamio.VarintSize(500)
 	)
 
-	var labelsSize int
-	for _, stream := range s.ordered {
-		for _, lbl := range stream.Labels {
-			labelsSize += len(lbl.Value)
-		}
-	}
-
 	var sizeEstimate int
 
 	sizeEstimate += len(s.ordered) * idDeltaSize        // ID
 	sizeEstimate += len(s.ordered) * timestampDeltaSize // Min timestamp
 	sizeEstimate += len(s.ordered) * timestampDeltaSize // Max timestamp
 	sizeEstimate += len(s.ordered) * rowDeltaSize       // Rows
-	sizeEstimate += labelsSize / 2                      // All labels (2x compression ratio)
+	sizeEstimate += s.currentLabelsSize / 2             // All labels (2x compression ratio)
 
 	return sizeEstimate
 }
@@ -151,6 +148,10 @@ func (s *Streams) addStream(hash uint64, streamLabels labels.Labels) *Stream {
 	// ordering.
 	sort.Sort(streamLabels)
 
+	for _, lbl := range streamLabels {
+		s.currentLabelsSize += len(lbl.Value)
+	}
+
 	newStream := &Stream{ID: s.lastID.Add(1), Labels: streamLabels}
 	s.lookup[hash] = append(s.lookup[hash], newStream)
 	s.ordered = append(s.ordered, newStream)
@@ -333,6 +334,7 @@ func (s *Streams) Reset() {
 	s.lastID.Store(0)
 	clear(s.lookup)
 	s.ordered = s.ordered[:0]
+	s.currentLabelsSize = 0
 	s.globalMinTimestamp = time.Time{}
 	s.globalMaxTimestamp = time.Time{}