diff --git a/src/lib/components/player/VideoPlayer.svelte b/src/lib/components/player/VideoPlayer.svelte
index 41f72f1e..0a320b70 100644
--- a/src/lib/components/player/VideoPlayer.svelte
+++ b/src/lib/components/player/VideoPlayer.svelte
@@ -12,6 +12,7 @@
   import SleepTimerModal from "./SleepTimerModal.svelte";
   import SleepTimerIndicator from "./SleepTimerIndicator.svelte";
   import CachedImage from "../common/CachedImage.svelte";
+  import { videoFitClass } from "./videoFit";
   import { sleepTimerActive, sleepTimerExpiredSignal } from "$lib/stores/sleepTimer";
   import { playbackPosition } from "$lib/stores/player";
   import * as html5Adapter from "$lib/player/html5Adapter";
@@ -1595,7 +1596,7 @@
       <video
           bind:this={videoElement}
           src={currentStreamUrl.includes('.m3u8') && Hls.isSupported() ? '' : currentStreamUrl}
-          class="max-w-full max-h-full"
+          class={videoFitClass()}
           class:invisible={!isMediaReady}
           style="filter: brightness({brightness})"
           playsinline
diff --git a/src/lib/components/player/videoFit.test.ts b/src/lib/components/player/videoFit.test.ts
new file mode 100644
index 00000000..82f055dd
--- /dev/null
+++ b/src/lib/components/player/videoFit.test.ts
@@ -0,0 +1,57 @@
+import { describe, it, expect } from "vitest";
+import { videoFitClass, fittedVideoSize } from "./videoFit";
+
+describe("videoFitClass", () => {
+  it("fills the container instead of capping at the source's intrinsic size", () => {
+    const cls = videoFitClass();
+    // max-w/max-h only shrink oversized media; a 480p source would stay a small
+    // box in the middle of a large window.
+    expect(cls).not.toContain("max-w-full");
+    expect(cls).not.toContain("max-h-full");
+    expect(cls).toContain("w-full");
+    expect(cls).toContain("h-full");
+  });
+
+  it("preserves aspect ratio while fitting (letterbox, never crop)", () => {
+    const cls = videoFitClass();
+    expect(cls).toContain("object-contain");
+    expect(cls).not.toContain("object-cover");
+    expect(cls).not.toContain("object-fill");
+  });
+});
+
+describe("fittedVideoSize", () => {
+  it("scales a 480p source up to fill a larger window (the reported bug)", () => {
+    // Exact 16:9 480p in a 1920x1080 window -> scales up to fill, rather than
+    // staying a 854x480 box in the middle.
+    const size = fittedVideoSize(853.33, 480, 1920, 1080);
+    expect(size.width).toBeCloseTo(1920, 0);
+    expect(size.height).toBeCloseTo(1080, 0);
+  });
+
+  it("fits to the constraining dimension when aspect ratios differ", () => {
+    // 4:3 source in a 16:9 window -> height-constrained, pillarboxed.
+    const size = fittedVideoSize(640, 480, 1920, 1080);
+    expect(size.height).toBeCloseTo(1080, 0);
+    expect(size.width).toBeCloseTo(1440, 0);
+    expect(size.width).toBeLessThan(1920);
+  });
+
+  it("fits to width when the source is wider than the window", () => {
+    // 21:9 source in a 16:9 window -> width-constrained, letterboxed.
+    const size = fittedVideoSize(2560, 1080, 1920, 1080);
+    expect(size.width).toBeCloseTo(1920, 0);
+    expect(size.height).toBeCloseTo(810, 0);
+    expect(size.height).toBeLessThan(1080);
+  });
+
+  it("shrinks oversized media to fit rather than overflowing", () => {
+    const size = fittedVideoSize(3840, 2160, 1280, 720);
+    expect(size.width).toBeCloseTo(1280, 0);
+    expect(size.height).toBeCloseTo(720, 0);
+  });
+
+  it("returns a zero size for unknown intrinsic dimensions", () => {
+    expect(fittedVideoSize(0, 0, 1920, 1080)).toEqual({ width: 0, height: 0 });
+  });
+});
diff --git a/src/lib/components/player/videoFit.ts b/src/lib/components/player/videoFit.ts
new file mode 100644
index 00000000..1355d863
--- /dev/null
+++ b/src/lib/components/player/videoFit.ts
@@ -0,0 +1,49 @@
+// Sizing rules for the HTML5 <video> element in the full-screen player.
+// Extracted from VideoPlayer.svelte so the fit behaviour is unit-testable.
+
+/**
+ * Classes applied to the <video> element so it fits the player viewport.
+ *
+ * TRACES: UR-005
+ *
+ * `max-w-full max-h-full` only ever *shrinks* oversized media, so a source
+ * smaller than the window (e.g. 480p on a 1080p display) rendered at its
+ * intrinsic size - a small box in the middle of a black screen. Filling the
+ * container and letting `object-contain` do the scaling fits the picture to
+ * whichever axis constrains it, in both directions, preserving aspect ratio.
+ */
+export function videoFitClass(): string {
+  return "w-full h-full object-contain";
+}
+
+export interface FittedSize {
+  width: number;
+  height: number;
+}
+
+/**
+ * The rendered size of a video of the given intrinsic dimensions once it has
+ * been fitted into the container - i.e. scaled (up or down) so that it touches
+ * the container on its constraining axis, with the other axis letter/pillar
+ * boxed. Mirrors what `object-fit: contain` on a full-size element does.
+ */
+export function fittedVideoSize(
+  intrinsicWidth: number,
+  intrinsicHeight: number,
+  containerWidth: number,
+  containerHeight: number,
+): FittedSize {
+  if (intrinsicWidth <= 0 || intrinsicHeight <= 0) {
+    return { width: 0, height: 0 };
+  }
+
+  const scale = Math.min(
+    containerWidth / intrinsicWidth,
+    containerHeight / intrinsicHeight,
+  );
+
+  return {
+    width: intrinsicWidth * scale,
+    height: intrinsicHeight * scale,
+  };
+}