diff --git a/k8s/infra/monitoring/prometheus-values.yaml b/k8s/infra/monitoring/prometheus-values.yaml
index 71efaae..c340397 100644
--- a/k8s/infra/monitoring/prometheus-values.yaml
+++ b/k8s/infra/monitoring/prometheus-values.yaml
@@ -55,7 +55,10 @@ alertmanager:
 prometheus:
   prometheusSpec:
     retention: 30d
-    retentionSize: "95GB"
+    # Must stay under the PVC size below (25Gi). At 95GB the limit never
+    # triggered, the volume filled, and Prometheus crashlooped with
+    # "no space left on device" writing /prometheus/queries.active.
+    retentionSize: "18GB"
 
     # scrapeTimeout MUST be <= scrapeInterval or the operator refuses to generate
     # the Prometheus config ("scrapeTimeout greater than scrapeInterval") and no