Overall the icon sizes in Plasma 5.10 have been improved on HiDPI displays (e.g. the handles on plasmoids). But unfortunately, there is also a regression: The application launcher icon is now incorrectly sized. This has been correct before (in Plasma 5.9). Before it has been the same size as the application icons, now the size of the application launcher icon in the panel is smaller. I've attached a screenshot that demonstrates this problem.
Created attachment 105784 [details] Application launcher in Plasma 5.9
Created attachment 105785 [details] Plasma 5.10
This is intentional - panel icons now follow the setting in System Settings -> Icons -> Advanced -> Panel as maximum size.
(In reply to Eike Hein from comment #3) > This is intentional - panel icons now follow the setting in System Settings > -> Icons -> Advanced -> Panel as maximum size. Nice!