Well, the reason for the scale of the Exploration Set model/props is readily apparent; they were intended for a child's hands. And since I was not yet twelve with a slight build, they didn't seem too undersized at that time.
As for their faithfulness to the on screen props, it looks as if they were based a bit more upon the simplified designs of the Filmation animated series. And, at least to me, the Mego figures seemed to resemble their cartoon counterparts more than the features of the original actors, particularly with Kirk and Scotty. Spock and McCoy, I will admit, differed equally from their drawn forms as they did from Nimoy and Kelley. Even Filmation didn't go crazy with Spock's ears as Mego did and McCoy looked more like Carl Malden in profile with that "ski slope" schnozz.
Never the less, I loved them. Since Star Wars was still about 3 years in the future, the standard size for action figures were 12 inches (ala G.I. Joe) or 8 inches (like the rest of the Mego line and several smaller companies). Thus, a "Creature from the Black Lagoon" doll I had became a sapient aquatic alien. (Doing a bit of 'net "research" not too long ago, I discovered this particular "Lagoon Creature" having "pop together" roto-cast torso and head parts along with quasi "bendy" arms and legs was meant by the manufacturers to be a female
creature, a gill-girl
, if you will! They did not feminize the face at all, but that does explain why it had a somewhat "hour-glass" shaped torso.)