As a rule of thumb, you can say that Vmca and Vmcg are more restrictive with a higher DA (density altitude). So the higher the altitude, or the higher the temperature, the higher the minimum control speeds will be.
In plain language, you could say that with a higher elevation or temperature, air is thiner, so a smaller mass of air is going by the rudder, and this makes the rudder less effective. To have the same mass of air passing by the rudder, you need to increase speed.
Hope I was helpful and good luck with your exams.