diff --git a/cpp/src/arrow/util/tdigest.cc b/cpp/src/arrow/util/tdigest.cc
index 36a83fb336d..b8e96b5f3fe 100644
--- a/cpp/src/arrow/util/tdigest.cc
+++ b/cpp/src/arrow/util/tdigest.cc
@@ -168,8 +168,8 @@ class TDigest::TDigestImpl {
     if (total_weight != total_weight_) {
       return Status::Invalid("tdigest total weight mismatch");
     }
-    // check if buffer expanded
-    if (tdigests_[0].capacity() > delta_ || tdigests_[1].capacity() > delta_) {
+    // check if buffer size exceeded delta
+    if (tdigests_[0].size() > delta_ || tdigests_[1].size() > delta_) {
       return Status::Invalid("oversized tdigest buffer");
     }
     // check k-size