I am currently playing as Melkor, a young warlock who uses his eldritch blast in pretty much every fight.
Since Melkor is level 7, he has access to 4 eldritch invocations. One of them is Repelling Blast, which is described as follows:
Prerequisite: eldritch blast cantrip
When you hit a creature with eldritch blast, you can push the creature up to 10 feet away from you in a straight line.
Recently, Melkor and his group have encountered a Hill Giant. A hill giant is huge. During the fight, it had a hard time getting close to the group, being pushed 20 feet per round (2 blasts).
My DM was not really happy with the fact that my medium human warlock was able to simply "shoo away" his giant. He seemed to say that it was not logical that in the event of me being near a dragon turtle, I could push it away with a boosted cantrip.
So, my question is: How does the eldritch invocation "Repelling Blast" affect a bigger creature?
Is there a general rule for this kind of spell?