Terms aggregation size may be 0.

"If set to 0, the size will be set to Integer.MAX_VALUE"

Aggregation/TermsAggregation.php

@@ -148,7 +148,7 @@ class TermsAggregation extends AbstractAggregation
             ];
         }
 
-        if ($this->size) {
+        if ($this->size !== null) {
             $data['size'] = $this->size;
         }