diff --git a/components/button/button.tsx b/components/button/button.tsx
index ff034502bb..9bc2b2423f 100644
--- a/components/button/button.tsx
+++ b/components/button/button.tsx
@@ -177,12 +177,8 @@ const InternalButton: React.ForwardRefRenderFunction<unknown, ButtonProps> = (pr
   };
 
   // =============== Update Loading ===============
-  let loadingOrDelay: Loading;
-  if (typeof loading === 'object' && loading.delay) {
-    loadingOrDelay = loading.delay || true;
-  } else {
-    loadingOrDelay = !!loading;
-  }
+  const loadingOrDelay: Loading =
+    typeof loading === 'object' && loading.delay ? loading.delay || true : !!loading;
 
   React.useEffect(() => {
     clearTimeout(delayTimeoutRef.current);
@@ -222,19 +218,9 @@ const InternalButton: React.ForwardRefRenderFunction<unknown, ButtonProps> = (pr
   const prefixCls = getPrefixCls('btn', customizePrefixCls);
   const autoInsertSpace = autoInsertSpaceInButton !== false;
 
-  // large => lg
-  // small => sm
-  let sizeCls = '';
-  switch (customizeSize || size) {
-    case 'large':
-      sizeCls = 'lg';
-      break;
-    case 'small':
-      sizeCls = 'sm';
-      break;
-    default:
-      break;
-  }
+  const sizeClassNameMap = { large: 'lg', small: 'sm', middle: undefined };
+  const sizeFullname = customizeSize || size;
+  const sizeCls = sizeFullname ? sizeClassNameMap[sizeFullname] || '' : '';
 
   const iconType = innerLoading ? 'loading' : icon;