diff --git a/scripts/moduleReport.ts b/scripts/moduleReport.ts
index 7dc738f345..394eb7e8e3 100644
--- a/scripts/moduleReport.ts
+++ b/scripts/moduleReport.ts
@@ -1,4 +1,6 @@
 import esbuild from 'esbuild';
+import path from 'path';
+import { explore } from 'source-map-explorer';
 
 // List of all modules accepted in ModulesMap
 const moduleNames = [
@@ -35,8 +37,14 @@ function formatBytes(bytes: number) {
   return `${formatted} KiB`;
 }
 
-// Gets the bundled size in bytes of an array of named exports from 'ably/modules'
-function getImportSize(modules: string[]) {
+interface BundleInfo {
+  byteSize: number;
+  code: Uint8Array;
+  sourceMap: Uint8Array;
+}
+
+// Uses esbuild to create a bundle containing the named exports from 'ably/modules'
+function getBundleInfo(modules: string[]): BundleInfo {
   const outfile = modules.join('');
   const result = esbuild.buildSync({
     stdin: {
@@ -48,9 +56,36 @@ function getImportSize(modules: string[]) {
     bundle: true,
     outfile,
     write: false,
+    sourcemap: 'external',
   });
 
-  return result.metafile.outputs[outfile].bytes;
+  const pathHasBase = (component: string) => {
+    return (outputFile: esbuild.OutputFile) => {
+      return path.parse(outputFile.path).base === component;
+    };
+  };
+
+  const codeOutputFile = result.outputFiles.find(pathHasBase(outfile))!;
+  const sourceMapOutputFile = result.outputFiles.find(pathHasBase(`${outfile}.map`))!;
+
+  return {
+    byteSize: result.metafile.outputs[outfile].bytes,
+    code: codeOutputFile.contents,
+    sourceMap: sourceMapOutputFile.contents,
+  };
+}
+
+// Gets the bundled size in bytes of an array of named exports from 'ably/modules'
+function getImportSize(modules: string[]) {
+  const bundleInfo = getBundleInfo(modules);
+  return bundleInfo.byteSize;
+}
+
+async function runSourceMapExplorer(bundleInfo: BundleInfo) {
+  return explore({
+    code: Buffer.from(bundleInfo.code),
+    map: Buffer.from(bundleInfo.sourceMap),
+  });
 }
 
 function printAndCheckModuleSizes() {
@@ -112,11 +147,96 @@ function printAndCheckFunctionSizes() {
   return errors;
 }
 
-(function run() {
+// Performs a sense check that there are no unexpected files making a large contribution to the BaseRealtime bundle size.
+async function checkBaseRealtimeFiles() {
+  const baseRealtimeBundleInfo = getBundleInfo(['BaseRealtime']);
+  const exploreResult = await runSourceMapExplorer(baseRealtimeBundleInfo);
+
+  const files = exploreResult.bundles[0].files;
+  delete files['[sourceMappingURL]'];
+  delete files['[unmapped]'];
+  delete files['[EOLs]'];
+
+  const thresholdBytes = 100;
+  const filesAboveThreshold = Object.entries(files).filter((file) => file[1].size >= thresholdBytes);
+
+  // These are the files that are allowed to contribute >= `threshold` bytes to the BaseRealtime bundle.
+  //
+  // The threshold is chosen pretty arbitrarily. There are some files (e.g. presencemessage.ts) whose bulk should not be included in the BaseRealtime bundle, but which make a small contribution to the bundle (probably because we make use of one exported constant or something; I haven’t looked into it).
+  const allowedFiles = new Set([
+    'src/common/constants/HttpStatusCodes.ts',
+    'src/common/constants/TransportName.ts',
+    'src/common/constants/XHRStates.ts',
+    'src/common/lib/client/auth.ts',
+    'src/common/lib/client/baseclient.ts',
+    'src/common/lib/client/baserealtime.ts',
+    'src/common/lib/client/channelstatechange.ts',
+    'src/common/lib/client/connection.ts',
+    'src/common/lib/client/connectionstatechange.ts',
+    'src/common/lib/client/realtimechannel.ts',
+    'src/common/lib/transport/connectionerrors.ts',
+    'src/common/lib/transport/connectionmanager.ts',
+    'src/common/lib/transport/messagequeue.ts',
+    'src/common/lib/transport/protocol.ts',
+    'src/common/lib/transport/transport.ts',
+    'src/common/lib/types/devicedetails.ts',
+    'src/common/lib/types/errorinfo.ts',
+    'src/common/lib/types/message.ts',
+    'src/common/lib/types/protocolmessage.ts',
+    'src/common/lib/types/pushchannelsubscription.ts', // TODO why? https://github.com/ably/ably-js/issues/1506
+    'src/common/lib/util/defaults.ts',
+    'src/common/lib/util/eventemitter.ts',
+    'src/common/lib/util/logger.ts',
+    'src/common/lib/util/multicaster.ts',
+    'src/common/lib/util/utils.ts',
+    'src/platform/web/config.ts',
+    'src/platform/web/lib/http/http.ts',
+    'src/platform/web/lib/util/bufferutils.ts',
+    'src/platform/web/lib/util/defaults.ts',
+    'src/platform/web/lib/util/hmac-sha256.ts',
+    'src/platform/web/lib/util/webstorage.ts',
+    'src/platform/web/modules.ts',
+  ]);
+
+  const errors: Error[] = [];
+
+  // Check that no files other than those allowed above make a large contribution to the bundle size
+  for (const file of filesAboveThreshold) {
+    if (!allowedFiles.has(file[0])) {
+      errors.push(
+        new Error(
+          `Unexpected file ${file[0]}, contributes ${file[1].size}B to BaseRealtime, more than allowed ${thresholdBytes}B`
+        )
+      );
+    }
+  }
+
+  // Check that there are no stale entries in the allowed list
+  for (const allowedFile of Array.from(allowedFiles)) {
+    const file = files[allowedFile];
+
+    if (file) {
+      if (file.size < thresholdBytes) {
+        errors.push(
+          new Error(
+            `File ${allowedFile} contributes ${file.size}B, which is less than the expected minimum of ${thresholdBytes}B. Remove it from the \`allowedFiles\` list.`
+          )
+        );
+      }
+    } else {
+      errors.push(new Error(`File ${allowedFile} is referenced in \`allowedFiles\` but does not contribute to bundle`));
+    }
+  }
+
+  return errors;
+}
+
+(async function run() {
   const errors: Error[] = [];
 
   errors.push(...printAndCheckModuleSizes());
   errors.push(...printAndCheckFunctionSizes());
+  errors.push(...(await checkBaseRealtimeFiles()));
 
   if (errors.length > 0) {
     for (const error of errors) {