Still hardcode UC20 amd64 image size, but to the same value as for UC16 and UC18.

debian/changelog

@@ -1,3 +1,12 @@
+livecd-rootfs (2.706) UNRELEASED; urgency=medium
+
+  * As per discussion, we might want to keep the UC20 images a bit bigger than
+    what's defined via the gadget/rootfs contents, to make sure writable is
+    comfortably big enough. Use the same hard-coded value as for UC16 and UC18.
+    (LP: #1905990)
+
+ -- Łukasz 'sil2100' Zemczak <lukasz.zemczak@ubuntu.com>  Thu, 03 Dec 2020 12:02:32 +0100
+
 livecd-rootfs (2.705) hirsute; urgency=medium
 
   * config: on hirsute build UC22 images

live-build/auto/config

@@ -361,11 +361,8 @@ case $IMAGEFORMAT in
 			case $MODEL in
 				pc-amd64|pc-i386)
 					if [ -z "${SUBARCH:-}" ]; then
-						case $SUITE in
-							xenial|bionic)
-								UBUNTU_IMAGE_ARGS="$UBUNTU_IMAGE_ARGS --image-size 3700M"
-								;;
-						esac
+						# This is to make sure there's enough writable space
+						UBUNTU_IMAGE_ARGS="$UBUNTU_IMAGE_ARGS --image-size 3700M"
 					fi
 					;;
 				*) ;;