Installing emacs-editing-major-modes wants to install 1 GiB of
packages... because it depends on emacs modes packages that themselves
are not big, but that themselves depend or recommend the
implementation for the language whose syntax they fontify.

This renders the package rather not useful to install blindly to "get
all major emacs editing modes". This package should at most suggest
any such emacs mode package. Feel free to instead convince the package
maintainer of the emacs mode package to suggest their language
implementation instead of recommending or depending on it.

In terms of size, elpa-haskell-mode is by far the main culprit, but
others include elpa-gnuplot-mode elpa-agda2-mode elpa-markdown-mode

Hello,

I don't think we should change emacs-editing-major-modes here.

Instead we should consider changing those other modes.  Or possibly do
nothing: maybe things are better overall as they are.

Sean Whitton <spwhitton@spwhitton.name> writes:

+1.  emacs-editing-major-modes is provided so that Emacs on Debian
supports most known formats out-of-the-box, and recommending available
major modes in Debian is by design.
installed tools and the editing modes should still work.

Lionel: please feel free to file wishlist bugs modes that "Depends" on
the tools to downgrade to using recommends, such as the specific mode
packages you listed.

On the other hand, as emacs-editing-major-modes only recommends the
corresponding major modes, as a workaround you are free to skip
installing the modes you don't like.  Personally I skip elpa-agda2-mode
due to the same reason as you.  Also, I use aptitude to selectively
exclude them, which can be done easier than using apt.
-- 
Regards,
Xiyue Deng