diff --git a/src/SparkRenderer.ts b/src/SparkRenderer.ts
index 7771c0c8..e10bab8f 100644
--- a/src/SparkRenderer.ts
+++ b/src/SparkRenderer.ts
@@ -808,7 +808,13 @@ export class SparkRenderer extends THREE.Mesh {
     // Determine render target
     const currentRenderTarget = renderer.getRenderTarget();
     const isXRRenderTarget = checkIsXRRenderTarget(currentRenderTarget);
-    if (currentRenderTarget) {
+    // Objects like Reflector reset the renderer's current viewport, so prefer
+    // the camera's own viewport.
+    const viewport =
+      camera.viewport ?? renderer.getCurrentViewport(new THREE.Vector4());
+    if (viewport.z > 1 && viewport.w > 1) {
+      spark.renderSize.set(viewport.z, viewport.w);
+    } else if (currentRenderTarget) {
       spark.renderSize.set(
         currentRenderTarget.width,
         currentRenderTarget.height,
diff --git a/test/unit/SparkRenderer.test.ts b/test/unit/SparkRenderer.test.ts
index 5f1599fc..46748786 100644
--- a/test/unit/SparkRenderer.test.ts
+++ b/test/unit/SparkRenderer.test.ts
@@ -20,6 +20,7 @@ const stubRenderer = {
   getActiveMipmapLevel: () => 0,
   setRenderTarget: () => {},
   getDrawingBufferSize: (size: THREE.Vector2) => size.set(1, 1),
+  getCurrentViewport: (viewport: THREE.Vector4) => viewport.set(0, 0, 1, 1),
   readRenderTargetPixelsAsync: async () => {},
   render: () => {},
   properties: new WeakMap(),
@@ -201,6 +202,36 @@ test("a change during a sort is sorted after it", async () => {
   expect(sorts.map((s) => s.version)).toEqual([v0 + 1, v0 + 2]);
 });
 
+test("a WebXR eye is sized by its viewport, not the shared render target", async () => {
+  const { spark, camera, frame } = await setup();
+  vi.spyOn(stubRenderer, "getRenderTarget").mockReturnValue({
+    width: 200,
+    height: 100,
+    isXRRenderTarget: true,
+    texture: { colorSpace: THREE.SRGBColorSpace },
+  } as unknown as THREE.WebGLRenderTarget);
+  // As after a Reflector renders in this eye
+  vi.spyOn(stubRenderer, "getCurrentViewport").mockImplementation((viewport) =>
+    viewport.set(0, 0, 200, 100),
+  );
+  camera.viewport = new THREE.Vector4(100, 0, 100, 100);
+
+  await frame();
+
+  expect(spark.renderSize).toEqual(new THREE.Vector2(100, 100));
+});
+
+test("a camera drawn with setViewport is sized by that viewport", async () => {
+  const { spark, frame } = await setup();
+  vi.spyOn(stubRenderer, "getCurrentViewport").mockImplementation((viewport) =>
+    viewport.set(50, 0, 50, 40),
+  );
+
+  await frame();
+
+  expect(spark.renderSize).toEqual(new THREE.Vector2(50, 40));
+});
+
 describe("a LoD tree that fails to initialize", () => {
   async function render(...lodTrees: unknown[]) {
     const { spark, frame } = await setup(