20Mb. Is pretty unreasonable. GTAV requires each vehicle to have a .yft and a _hi.yft for the vehicle files and a .ytd for the texture dictionary. Depending on the base, one vehicle file size can get close to that 20Mb. just on its own. Sure, some compression can be used to reduce the number of materials by reusing some or deleting unused ones, However, most vehicles can't get much smaller without reducing the poly count (which is hard and painful) and ruins the visual fidelity of the model. Most vehicles, bases, and items used are optimized the best they can be without ruining the model. You also, can't really compress a .ytd either. DDS images are already pretty small leaving no room for any additional optimization. You can reuse textures, but in the end, you need to have some textures and they need to retain some quality. I have over five-hundred vehicle mods installed in my game and I have no issues. Vehicles will only be rendered when on-screen or loading in. Most vehicles in my game with all their files combined are around 50Mb. each. I am not saying 20Mb. isn't possible, but it would require a lot of extra work, additional programs (Which can cost thousands of dollars), and the loss of overall visual accuracy that someone worked so hard on. In the end, not every mod needs to be painstakingly optimized so people with low-end machines can use them.
Their activity is the worse. Faceless claims, toxicity, begging, criticism based on opinions, and other garbage. So I think that is just who they are. Complain about everything without knowing how much money and effort goes into things.