Comment 3 for bug 1790134

The problem is fixed in Cosmic/Disco/Eoan, probably because yaru provides an icon with the correct geometry, it might still be a problem with other themes though.

One simply change like

--- UpdateManager.ui 2018-10-16 16:11:19.117279284 +0200
+++ UpdateManager.ui.new 2018-12-07 10:18:05.840037377 +0100
@@ -182,6 +182,7 @@
                 <property name="visible">True</property>
                 <property name="can_focus">False</property>
                 <property name="icon_name">aptdaemon-download</property>
+ <property name="pixel-size">16</property>
               </object>
               <packing>
                 <property name="expand">False</property>

should be enough to resolve the issue on bionic/with other theme though, if someone wants to probably test and submit a mr for it (note that the battery icon seems to have the same issue)