diff --git a/py/services/model_update_service.py b/py/services/model_update_service.py
index 9df0dbeb..dabe4ad4 100644
--- a/py/services/model_update_service.py
+++ b/py/services/model_update_service.py
@@ -930,18 +930,39 @@ class ModelUpdateService:
     def _extract_size_bytes(self, files) -> Optional[int]:
         if not isinstance(files, Iterable):
             return None
+
+        def parse_size(entry: Mapping) -> Optional[int]:
+            size_kb = entry.get("sizeKB")
+            if size_kb is None:
+                return None
+            try:
+                return int(float(size_kb) * 1024)
+            except (TypeError, ValueError):
+                return None
+
+        preferred_size: Optional[int] = None
+        fallback_size: Optional[int] = None
         for entry in files:
             if not isinstance(entry, Mapping):
                 continue
-            size_kb = entry.get("sizeKB")
-            if size_kb is None:
+            size_bytes = parse_size(entry)
+            if size_bytes is None:
                 continue
-            try:
-                size_float = float(size_kb)
-            except (TypeError, ValueError):
-                continue
-            return int(size_float * 1024)
-        return None
+
+            entry_type = entry.get("type")
+            is_model_type = isinstance(entry_type, str) and entry_type.lower() == "model"
+            primary_flag = entry.get("primary")
+            is_primary = primary_flag is True or (
+                isinstance(primary_flag, str) and primary_flag.strip().lower() == "true"
+            )
+
+            if is_model_type and is_primary:
+                preferred_size = size_bytes
+                break
+            if fallback_size is None:
+                fallback_size = size_bytes
+
+        return preferred_size if preferred_size is not None else fallback_size
 
     def _extract_preview_url(self, images) -> Optional[str]:
         if not isinstance(images, Iterable):
diff --git a/tests/services/test_model_update_service.py b/tests/services/test_model_update_service.py
index 5538f3b8..61799715 100644
--- a/tests/services/test_model_update_service.py
+++ b/tests/services/test_model_update_service.py
@@ -74,6 +74,58 @@ def make_record(*versions, should_ignore_model=False):
     )
 
 
+def test_extract_size_bytes_prefers_primary_model_file(tmp_path):
+    db_path = tmp_path / "updates.sqlite"
+    service = ModelUpdateService(str(db_path))
+
+    response = {
+        "modelVersions": [
+            {
+                "id": 42,
+                "files": [
+                    {"sizeKB": 2018.0400390625, "type": "Training Data", "primary": False},
+                    {
+                        "sizeKB": 1152322.3515625,
+                        "type": "Model",
+                        "primary": "True",
+                    },
+                ],
+                "images": [],
+            }
+        ]
+    }
+
+    versions = service._extract_versions(response)
+    assert versions is not None
+    assert versions[0].size_bytes == int(1152322.3515625 * 1024)
+
+
+def test_extract_size_bytes_falls_back_without_primary(tmp_path):
+    db_path = tmp_path / "updates.sqlite"
+    service = ModelUpdateService(str(db_path))
+
+    response = {
+        "modelVersions": [
+            {
+                "id": 43,
+                "files": [
+                    {
+                        "sizeKB": 2048,
+                        "type": "Training Data",
+                        "primary": True,
+                    },
+                    {"sizeKB": 1024, "type": "Archive", "primary": False},
+                ],
+                "images": [],
+            }
+        ]
+    }
+
+    versions = service._extract_versions(response)
+    assert versions is not None
+    assert versions[0].size_bytes == int(2048 * 1024)
+
+
 def test_has_update_requires_newer_version_than_library():
     record = make_record(
         make_version(5, in_library=True),