Not sure what's causing this.
Some icons are boundled within inkscape, some came from gtk, and the display of them is also affected by os settings -at least on win 7-8 (?).
Probably the developers are working on it, as it seems appearing at most users, just as
lagging.
smaller "affects" iconstall toolbarstock system icons overridingcommand bar iconslayer padlock iconinskcape icons.svg loading brokena bit related:
custom themeand finally -in this case I would check the mentioned setting first:
icon size affected by display settingsThere are quite some topics on the icons in general here, so adding tags for a better search.
icon missing theme dark 64 gtk large small blurred command bar interface high dpi properties compatibility