What you are seeing is probably caused by the activity of appstream.
See https://www.freedesktop.org/software/appstream/docs/sect-AppStream-IconCache.html or https://askubuntu.com/questions/775069/what-is-appstream-why-is-it-installed-what-is-it-doing.
With appstream installed, sudo apt update would produce something like this (on Xubuntu 18.10):
Ign:1 http://dl.google.com/linux/chrome/deb stable InRelease Get:2 http://security.ubuntu.com/ubuntu cosmic-security InRelease [83.2 kB] Hit:3 http://in.archive.ubuntu.com/ubuntu cosmic InRelease Get:4 http://in.archive.ubuntu.com/ubuntu cosmic-updates InRelease [83.2 kB] Get:5 http://dl.google.com/linux/chrome/deb stable Release [943 B] Get:6 http://dl.google.com/linux/chrome/deb stable Release.gpg [819 B] Hit:7 http://in.archive.ubuntu.com/ubuntu cosmic-backports InRelease Get:8 http://security.ubuntu.com/ubuntu cosmic-security/main i386 Packages [33.4 kB] Get:9 http://in.archive.ubuntu.com/ubuntu cosmic-updates/main i386 Packages [47.7 kB] Get:10 http://security.ubuntu.com/ubuntu cosmic-security/main amd64 Packages [33.4 kB] Get:11 http://in.archive.ubuntu.com/ubuntu cosmic-updates/main amd64 Packages [49.4 kB] Get:12 http://security.ubuntu.com/ubuntu cosmic-security/main Translation-en [13.7 kB] Get:13 http://in.archive.ubuntu.com/ubuntu cosmic-updates/main Translation-en [20.2 kB] Get:14 http://security.ubuntu.com/ubuntu cosmic-security/main amd64 DEP-11 Metadata [204 B] Get:15 http://in.archive.ubuntu.com/ubuntu cosmic-updates/main amd64 DEP-11 Metadata [52.0 kB] Get:16 http://security.ubuntu.com/ubuntu cosmic-security/main DEP-11 48x48 Icons [29 B] Get:17 http://security.ubuntu.com/ubuntu cosmic-security/main DEP-11 64x64 Icons [29 B] Get:18 http://security.ubuntu.com/ubuntu cosmic-security/universe amd64 Packages [22.2 kB] Get:19 http://in.archive.ubuntu.com/ubuntu cosmic-updates/main DEP-11 48x48 Icons [5,926 B] Get:20 http://in.archive.ubuntu.com/ubuntu cosmic-updates/main DEP-11 64x64 Icons [8,457 B] Get:21 http://security.ubuntu.com/ubuntu cosmic-security/universe i386 Packages [22.2 kB] Get:22 http://in.archive.ubuntu.com/ubuntu cosmic-updates/universe i386 Packages [29.9 kB] Get:23 http://in.archive.ubuntu.com/ubuntu cosmic-updates/universe amd64 Packages [29.9 kB] Get:24 http://security.ubuntu.com/ubuntu cosmic-security/universe Translation-en [11.8 kB] Get:25 http://in.archive.ubuntu.com/ubuntu cosmic-updates/universe Translation-en [15.4 kB] Get:26 http://security.ubuntu.com/ubuntu cosmic-security/universe amd64 DEP-11 Metadata [2,448 B] Get:27 http://in.archive.ubuntu.com/ubuntu cosmic-updates/universe amd64 DEP-11 Metadata [5,724 B] Get:28 http://security.ubuntu.com/ubuntu cosmic-security/universe DEP-11 48x48 Icons [4,031 B] Get:29 http://in.archive.ubuntu.com/ubuntu cosmic-updates/universe DEP-11 48x48 Icons [4,031 B] Get:30 http://security.ubuntu.com/ubuntu cosmic-security/universe DEP-11 64x64 Icons [4,058 B] Get:31 http://in.archive.ubuntu.com/ubuntu cosmic-updates/universe DEP-11 64x64 Icons [4,058 B] Reading package lists... Done
The downloads for icons in the example above are relatively small but can be several MB in distros which are in alpha or beta stage of development.
If, as you say,
I do not need to download icons as I am not using GUI package managers.
simply run sudo apt purge appstream and examine the output before proceeding:
$ sudo apt purge appstream* Reading package lists... Done Building dependency tree Reading state information... Done Note, selecting 'appstream-generator' for glob 'appstream*' Note, selecting 'appstream-glib-doc' for glob 'appstream*' Note, selecting 'appstream' for glob 'appstream*' Note, selecting 'appstream-util' for glob 'appstream*' Note, selecting 'appstream-index' for glob 'appstream*' Note, selecting 'appstream-doc' for glob 'appstream*' Note, selecting 'appstream-qt5' for glob 'appstream*' Note, selecting 'appstream' instead of 'appstream-index' Package 'appstream-qt5' is not installed, so not removed Package 'appstream-doc' is not installed, so not removed Package 'appstream-glib-doc' is not installed, so not removed Package 'appstream-generator' is not installed, so not removed Package 'appstream-util' is not installed, so not removed The following packages will be REMOVED: appstream* apt-config-icons* gnome-software* gnome-software-plugin-snap* 0 upgraded, 0 newly installed, 4 to remove and 0 not upgraded. After this operation, 3,029 kB disk space will be freed. Do you want to continue? [Y/n]
Of course, what you see on your systems maybe different.
appstream.