Bundle size for node_module firebase increases significantly when upgrading from Next 14 to 15

[MaxPuckeridge]

Link to the code that reproduces this issue

https://github.com/MaxPuckeridge/nextjs-bundle-issue

To Reproduce

I've made two versions of a nextjs app.

The original was made using npx create-next-app@14 in the oldee folder. I added a very basic import of a firebase utility to make sure it is pulled into the build.

I then made a copy of this app, in the newee folder, and then ran npx @next/codemod upgrade saying yes to most options, including react 19.

There is @next/bundle-analyzer for both, and the outputs of both are in the outputs/ folder in the root of the repo.

There is a noticable change in firestore between the two nextjs versions. The main difference seems to be the gzipped version, 41 vs 70kb

Current vs. Expected behavior

Ideally the size of our node_modules should stay the same or be better. A regression to bundle size is not ideal

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.2.0: Fri Dec  6 19:01:59 PST 2024; root:xnu-11215.61.5~2/RELEASE_ARM64_T6000
  Available memory (MB): 16384
  Available CPU cores: 10
Binaries:
  Node: 20.18.3
  npm: 10.8.2
  Yarn: 1.22.15
  pnpm: 8.15.3
Relevant Packages:
  next: 15.2.3 // Latest available version is detected (15.2.3).
  eslint-config-next: 15.2.3
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.8.2
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Performance

Which stage(s) are affected? (Select all that apply)

next build (local), Other (Deployed), Vercel (Deployed)

Additional context

No response

[GusRuss89]

This is preventing us from upgrading to v15. Can anyone explain why it's affecting the bundle size of another package so much?

[kdy1]

Can you try some older version of next@v15? webpack/webpack#19184 seems to be the cause.