Despite the classifying name 'mecha', the references for the mecha scale were far more inspired from other vehicles, especially the flying ones.
Light Mecha's reference point was the shuttlepod, Medium mecha is for larger shuttles and fightercraft, while Heavy Mecha was meant for much larger craft, with the NASA Orbiter shuttle being the benchmark for it.
Earlier draft actually had this category called 'Vehicle' (as in, Anti-Vehicle), but it was argued to me that mecha ought to still fit because it was short for Mechanics/mechanical - which, as you'll observe if you eye the homepages of various sci-fi animes, can apply for tanks just as much as planes, shuttles and giant robots. So, I conceded the point and left it as "Mecha".
What you need to take away from that is that it doesn't only cover the big robots. As for the drastic scale increase between Heavy mecha and Light starship, so far as I was concerned it was as designed. Growing the heavy mecha more turned them into mini-spaceships, which did not seem desirable regarding what was already established in the setting. Thematically, SARP had a sharp constrast when it came to other things and then ships. Having the mecha tiers alone actually made ships far more assailable than they were before.
If after that you still beleive the values listed are a mistake, you need to propose replacement numbers and run them through Wes to get his approval for it. For my part, I can't answer for what the mods let through: I'm not one.