The Die Cast vehicles are a completely separate line and are larger than the plastic 3-pack vehicles. They are as different from each other as they are from the Action Fleet line. There's a nice catalogue of the Micro Machines items in #118 of Lee's Toy Review and if you're a fan of the lines then it's well worth picking up
the die cast line is the same size as the small ships included as part of the series alpha set.
also, the same molds that were used for the die cast line, were also used for the x-ray line.
one of the amazing details given by galoob to their star wars line was that by making so many different sized ships, (die cast, action fleet, micro machines, etc.) a proper sized scale ship can be found from one of the lines that matches up in scale to a ship in another line.
the micro machines snowspeeder compares properly in size to the action fleet at-at
the die-cast x-wing compares properly in size to the action fleet millenium falcon
better yet, the micro machines rebel blockade runner (tantive VII) compares well toe the kenner collection series star destroyer.
there are a lot more favorable size match ups through the lines.
the micro machines desert skiffs are in relative scale to the action fleet jabba's sail barge.
the same way the mm snowspeeders are in scale to the action fleet at-at, so is the mm at-st
the mm lukes land speeder (along with the other speeders, lars family, etc.)are in scale with the af sand crawler
the mm play set of the rebel transport is somewhat scaled to the mm fighter ships (x-wing, tie fighters etc.)
the action fleet mtt is in scale with the mm gian speeder. not really knowing what size the att droid tank is supposed to be, i think that the att die-cast is in scale to the mtt (the mm tanks i think are a little too small.
there are more, but that's all i can think of right now off the top of my head