Normally I wouldnāt get involved in dicussions like this, since my typical philosphy is that in imaginary worlds, anything can happen and applying r/l scientific concepts is counter-intuitive. However in this instance the fact part of my degree was in cosmology I feel a driving desire to discuss this topic.
Ok, so what facts do we know about 3rd Earth. Well, it has the name Earth, however that dosnāt mean it is actaully Earth. It could potential just be the third planet to ever be named Earth, so that dosnāt actually make it Earth, just another planet named Earth.
What climates have we seen. Well, through the entirity of the show, we saw snow capped mountains (hook mountain), desert (black pyramid), open plains (quite alot actually), snowy landscapes (Ice Queen), swamps (plun-darr), jungle (the amazons) and quite a few others. So to summarise, weāve seen as many environments as there are in the real world, which does actaully suggest the planet is sphereical, the fact they have gravity means the planet should be spherical.
Quite well pointed out however is that in most episodes, the characters were never far from each other. Realistically, they were always way too close to one another, only in a few episodes did they ever actually have a proper time frame in play (the only episode that comes to mind at the moment, is the Technopeade episode with Turmagar and possibly queen of eight legs).
However this inconsistency can be slightly minimalised, in that mass is not proportional to size, so there is nothing to say that 3rd Earth is a class E planet (more than aware iām quoting Star Trek here, but unfortunatly their fictitious classification of planets is actually been taught these days as it works) like Earth. It could actually be a class A or B planet (making it less than 2000km in diameter or a circumference of 3905 miles, sounds alot but isnāt). Also, if we imagine that there is only one continent (a panjaea setup), then the Thundercats, berbils, mutants etc would be quite close to one another (but not exactly spitting distance) and in a vehicle like the mutant flyers or the Thunder-tank, covering distance wouldnāt be very hard. If the thundertank can do speeds of a modern day car, then at 60mph it would be able to drive around the equator of the planet in less than 65 hours.
Also just to be cheap, if we say that 3rd Earth is very dense, then thatās why it maintains a functional gravity field. Or to be really picky, we could just say that the sun in the solar system is much bigger than out sun, which would also increase the gravitational pull of the planet.
Not too sure what this post has acheived, but I feel better for actaully making a contribution.