I've heard people mention this a lot in how they rationalize the DCUC MOTUC difference... and if it works for you thats all that matters. For me I just think they look ridiculous ... and I don't like to have to rationalize why my figures look weird together. Its not as big of a deal with merman or beast man or characters that are creatures ... but putting the human characters next to each other they just look really out of scale.
The way I see it;
Eternians are all equally muscular and tall. A lot more than Earthlings (unless Earthlings live for over 10 years on Eternia, then they will grow taller as well *Marlena*). This is because of the gravity, the suns and the center of the Universe. ThunderCats are just a taller race of Cats from Thundera and the Mutants grow taller on Plundar. Both races have a standard/slim bodymass unlike a muscular body from the people of Eternia.