I don't think Mike's airtime is disproportionate to his jobs on the show. He functions as the host, the buyer, the seller, and the parts obtainer often with additional content depicting the actions or services of a local parts seller or tradesman/craftsman providing a service needed for that episode's car (upholstery, body work, wood dash restoration, metal refinishing, component rebuilding, etc). So it stands to reason that he will have more time than whomever the mechanic is.
You also have to remember that in today's TV world, an hour ain't as long as it used to be. For example, when I took the Motor Trend trial to see the 124 show ahead of time, I downloaded the episode. According to my playback software, the actual playing time of the episode is 43:38.
As far as the car mix is concerned, since what Wikipedia calls Series 13 (starting with episode 120) where all shows were from the SoCal workshop and all but one car was purchased in the USA, they have done 15 American cars and 29 foreign cars (I counted the VW Rabbit and the "Mercury" Capri as foreign.)