You can compare the git repos used to build the images (rootfs.manifest is useful). Or you can run each image and see what they show is different:
$ docker run --rm debian:stable dpkg --get-selections >debian-stable-pkgs.txt
$ docker run --rm debian:stable-slim dpkg --get-selections >debian-stable-slim-pkgs.txt
$ diff debian-stable-pkgs.txt debian-stable-slim-pkgs.txt
23,24d22
< iproute2 install
< iputils-ping install
35,36d32
< libcap2:amd64 install
< libcap2-bin install
40d35
< libelf1:amd64 install
53d47
< libmnl0:amd64 install
77d70
< libxtables12:amd64 install
Additionally, as Tomofumi points out, there are various files excluded from the image (with some others reincluded). These are mostly documentation and language support:
/usr/share/doc/*
/usr/share/info/*
/usr/share/linda/*
/usr/share/lintian/overrides/*
/usr/share/locale/*
/usr/share/man/*
/usr/share/doc/kde/HTML/*/*
/usr/share/gnome/help/*/*
/usr/share/locale/*
/usr/share/omf/*/*-*.emf
So by excluding a handful of packages, and stripping various docs and localization files, they were able to trim 45MB from the image, or about 40%.
$ docker image ls debian
REPOSITORY TAG IMAGE ID CREATED SIZE
debian stable-slim eb8569e750e6 2 weeks ago 69.2MB
debian stable 405289501bdf 2 weeks ago 114MB