The Sentry team reduced their JavaScript SDK's bundle size by 29% by identifying and removing unnecessary code generated during transpilation and downcompilation. They achieved this through various techniques, including removing optional chaining, switching from TypeScript enums to const and string enums, minifying JavaScript assets, using try-catch blocks to minify code requiring nested object access, aliasing object keys to local variables, and converting classes to objects and functions while minimizing private fields. These changes resulted in a more efficient bundle size, reducing the number of bytes taken up by the SDK's generated JavaScript.