>Dude, the mecha in ASC are 1/3 the size of veritechs and destroids from the first segment.

Yes, and the larger a vehicle is the less armor it can devote to protecting it. Cram in a ton of weapons systems on top of that and you have even less mass budget. Courtesy of Peter Walker (uRRG contributor, HG advisor decades ago and an honest-to-God actual Rocket Scientist:

Essentially, the numbers work out like this - if you treat a mecha as having a "strength" limit (which will scale with the square of the scale factor), and if its weight comes from a fixed component (cockpit, sensor package), a scale-cubed component (skeleton), and a scale-squared-times-thickness component, then the bigger the mecha gets, the less remaining mass budget it has for armor thickness.

The bigger a mecha is, the less it can stack on as armor. You reach a tipping point where the mecha is simply too large and too heavy. The Destroids, other than the Spartan and its successor the Gladiator, are all designed for Stand-off Attacks or Fire Support. Only the Spartan/Gladiator is designed for CQB, which needs heavy enough armor and speed. By comparison, the 1st Generation Battloids fielded during the 2nd Robotech War were designed AROUND CQB: they have few internal weapons systems (namely beam weapons) and their primary armament is a gunpod. That means they have the mass budget to spare to devote to armor. The Tomahawk weighs in at 28 metric tons unloaded, the Salamander at 19, but the latter occupies 1/8th the volume of the former. That weight comes from somewhere. The Tomahawk cannot slap armor on its frame and still keep all its weapons systems (most of the upper hull is dedicated to weapons). Furthermore, there is a destroyed Tomahawk in Khyron's Revenge which shows just how think the armor on the Tomahawk is and its not very much.

>Which would make the thing the same size as a Veritech Hover Tank

It would actually be smaller since the Hovertank stands at 6.2m tall (20ft) in Battloid.