The sizes of deb and rpm packages can differ greatly because the default compression levels for deb and rpm packages are different:

• RPMs use level 9
• DEBs use level 6 which is used by tar that creates data.tar.xz.

Unlike RPMs (--rpm-compression-level), there is no way to specify a compression level for DEBs.
This could be achieved by either adding an environment variable to tar, such as XZ_OPT for xz, or by first creating a tar archive and then compressing it.