Upgrade from jessie to strech wants to bloat by system

[Urs Thuermann <urs@isnogud.escape.de>]

After fully updating my jessie system using

        aptitude update; aptitude full-upgrade

I edited sources.list to dist-upgrade to strech.  A folloing aptitude
upgrade wants to install additional 1.5 GB on my system which is
currently ~5 GB, i.e. a 30% increase:

  # df -h / /usr
  Filesystem      Size  Used Avail Use% Mounted on
  /dev/dm-0       2.0G  1.8G  135M  93% /
  /dev/dm-2       7.8G  4.4G  3.0G  60% /usr
  # du -hs /root
  1.4G    /root
  # aptitude upgrade
  Resolving dependencies...
  ...
  The following NEW packages will be installed:
    btrfs-progs{a} clang-3.8{a} cpp-6{a} cpp-6-doc{a} dh-autoreconf{a}
    ...
  The following packages will be REMOVED:
    cpp-4.9-doc{u} docutils-common{u} docutils-doc{u} g++-4.9{u} gir1.2-vte-2.90{u}
    ...
  The following packages will be upgraded:
    acct acl acpi acpi-support-base acpid adduser adwaita-icon-theme apache2
    ...
  1151 packages upgraded, 297 newly installed, 128 to remove and 82 not upgraded.
  Need to get 1843 MB of archives. After unpacking 1537 MB will be used.
  Do you want to continue? [Y/n/?]

I see that some new versions of packages are installed without the old
versions being removed, although they are marked as automatically
installed, e.g. Linux kernel, clang, llvm, and some others.  For
example

  # aptitude search "~i clang"
  i   clang                     - C, C++ and Objective-C compiler (LLVM based)
  i A clang-3.5                 - C, C++ and Objective-C compiler (LLVM based)
  i A libclang-common-3.5-dev   - clang library - Common development package
  i A libclang1-3.5             - C interface to the clang library
    
and aptitude full-upgrade will install clang-3.8 but not remove
clang-3.5.  But my suspicion is that even when I manually remove all
these old packages, the installation is still unreasonably larger than
it is currently.

urs