The difference between the minimal and the normal installation options is just that canonical removed a few packages from being installed by default. The difference is fairly small, around 400mb in total installation size. You can simply remove the packages that you do not need with sudo apt remove PACKAGENAME or sudo apt purge PACKAGENAME.
To answer the question, here is a list of all the packages that have been removed if you install the 18.04 with the minimal option. A similar list for ubuntu-mate can be found here.
# This is a list of packages to be removed when the 'minimal' option is
# selected during installation.
# Note that the format is NOT the same as the usual seed format. This file is
# not processed by germinate - it is simply downloaded during image builds.
# Desktop apps
thunderbird
transmission-gtk
gnome-todo
baobab
rhythmbox
cheese
vino
shotwell
totem
usb-creator-gtk
deja-dup
gnome-calendar
remmina
simple-scan
# Desktop experience
thunderbird-gnome-support
# Games
aisleriot
gnome-mahjongg
gnome-mines
gnome-sudoku
branding-ubuntu
# Libreoffice
libreoffice-style-breeze
libreoffice-gnome
libreoffice-writer
libreoffice-calc
libreoffice-impress
libreoffice-math
libreoffice-ogltrans
libreoffice-pdfimport
# Others
example-content
ubuntu-web-launchers
# Langpacks (keeping libreoffice and thunderbird installed otherwise)
libreoffice-l10n-en-gb
libreoffice-l10n-es
libreoffice-l10n-zh-cn
libreoffice-l10n-zh-tw
libreoffice-l10n-pt
libreoffice-l10n-pt-br
libreoffice-l10n-de
libreoffice-l10n-fr
libreoffice-l10n-it
libreoffice-l10n-ru
libreoffice-l10n-en-za
libreoffice-help-en-gb
libreoffice-help-es
libreoffice-help-zh-cn
libreoffice-help-zh-tw
libreoffice-help-pt
libreoffice-help-pt-br
libreoffice-help-de
libreoffice-help-fr
libreoffice-help-it
libreoffice-help-ru
libreoffice-help-en-us
thunderbird-locale-en
thunderbird-locale-en-gb
thunderbird-locale-en-us
thunderbird-locale-es
thunderbird-locale-es-ar
thunderbird-locale-es-es
thunderbird-locale-zh-cn
thunderbird-locale-zh-hans
thunderbird-locale-zh-hant
thunderbird-locale-zh-tw
thunderbird-locale-pt
thunderbird-locale-pt-br
thunderbird-locale-pt-pt
thunderbird-locale-de
thunderbird-locale-fr
thunderbird-locale-it
thunderbird-locale-ru
# Unused rdepends
gir1.2-rb-3.0
gir1.2-totem-1.0
gir1.2-totemplparser-1.0
guile-2.0-libs
libabw-0.1-1
libavahi-ui-gtk3-0
libdmapsharing-3.0-2
libexttextcat-2.0-0
libexttextcat-data
libfreehand-0.1-1
libgnome-games-support-1-3
libgnome-games-support-common
libgom-1.0-0
libgrilo-0.3-0
liblangtag-common
liblangtag1
libmessaging-menu0
libmhash2
libminiupnpc10
libmwaw-0.3-3
libmythes-1.2-0
libnatpmp1
libneon27-gnutls
liborcus-0.13-0
libpagemaker-0.0-0
librdf0
libreoffice-avmedia-backend-gstreamer
libreoffice-base-core
libreoffice-common
libreoffice-core
libreoffice-draw
libreoffice-gtk3
libreoffice-style-elementary
libreoffice-style-galaxy
libreoffice-style-tango
libraptor2-0
librasqal3
librevenge-0.0-0
librhythmbox-core10
libtotem0
libvisio-0.1-1
libwpd-0.10-10
libwpg-0.3-3
libwps-0.4-4
libyajl2
python3-uno
rhythmbox-data
rhythmbox-plugin-alternative-toolbar
rhythmbox-plugins
remmina-common
remmina-plugin-rdp
remmina-plugin-secret
remmina-plugin-vnc
duplicity
seahorse-daemon
shotwell-common
totem-common
totem-plugins
transmission-common
cheese-common
gnome-todo-common
libgnome-todo
gnome-video-effects
libcheese-gtk25
libcheese8
uno-libs3
ure
zeitgeist-core
hunspell-de-at-frami
hunspell-de-ch-frami
hunspell-de-de-frami
hunspell-en-au
hunspell-en-ca
hunspell-en-gb
hunspell-en-za
hunspell-es
hunspell-fr
hunspell-fr-classical
hunspell-it
hunspell-pt-br
hunspell-pt-pt
hunspell-ru
hyphen-de
hyphen-en-ca
hyphen-en-gb
hyphen-en-us
hyphen-fr
hyphen-hr
hyphen-it
hyphen-pl
hyphen-pt-br
hyphen-pt-pt
hyphen-ru
mythes-de
mythes-de-ch
mythes-en-au
mythes-en-us
mythes-fr
mythes-it
mythes-pt-pt
mythes-ru
