With the new update the lines between equipment types have become more blurred. There are a lot more variables now. There are modules that work similarly for multiple types of equipment. For tanks you can create comparable light and medium tanks, at least earlier in the game. As the time goes on you are constricted by more advanced modules which require bigger frames. Something similar happens with planes. With the inclusion of double engines in the light frame setup you can create designs that bridge the gap between light and medium planes. The outcome is you get models that are more heavily armed than lights but lack the range or capabilites of mediums.
About the Italian tank: by 1939 afvs had increased in size a lot. An 11t tank such as the M11/39 which by the early thirties dwarfed other Regio Esercito tankettes, was not adequate anymore. The mainstay medium tanks of the time were as much as twice as large as it. Thats why its been given a light chasis. This also differentiates it from other, heavier Italian models.
The Bf 110 wasnt a heavy fighter per se. It had a slim fuselage and rather short wings. The plane was comperable in size to the 109. It came from german experimentation with the Schnellbomber and later Zerstörer program. Both gave birth to relatively light and fast combat aircraft. Another product of this line of thinking, the Do 17 is also ingame a light airframe. They stand in contrast to larger planes like the Ju 88 series, Do 217, Fw 200 which were propper long range planes.
All of this does not come without its issues. The Sprites and Models are assigned in the designer according to the old tech tree where the roles were set more arbitrarly. However im hopeful that the situation will improve with time.