The simplest explanation for Metrotitan's size is that he was from Zone-meaning he was a Micromaster base: i.e unlike Metroplex, who's toy was not an accurate portrayal of his size in the fiction (compared to other characters' toys), Metrotitan
was the size/scale of his toy in fiction.
Much like how Grand Maximus in Masterforce is toy size (with God Ginrai riding on him), Unlike fortress Maximus who was portrayed at TF city scale in Headmasters (with full sized characters like chromedome coming out of tiny doors in his foot), not his toy size.
And yes, I agree with Sabre. Omega was the same size as Devastator in the G1 toon. Combiners are big, but Metroplex is big enough to fit half to 3/4 of a combiner in his hallways (or near enough) A combiner would be nowhere near the size of Metroplex's leg even, much less higher, I'd say. Metroplex is an entire city for Full size TF's. Although that said combiners and Omega mass shifted to suit purpose thanks to wonky planning by writers and animators of season's 1-3. Look at Astrotrain for crying out load. In bot mode he is as tall as Megatron yet in shuttle mode he can carry 20 of him
so who knows.
I stand by my Metrotitan explanation though