You are still forgetting a lot of factors..
By using flak jacket, the player gives up a grenade as well as the health regeneration of not equipping Clotting Agent, which sadly the game revolves around & mostly gets balanced around. There is a lot to give up here just to walk away from explosions with a tiny bit of health that can be taken away by anything in the game.
What do players give up from not using Valziprams however? Either Happy Landings, Car Surfer or Hard Landings since that's all the blue character modifications in the game... I am sorry, but telling me that "it's not very used" is a flat out lie... press tab to check what mods players equip more often.. Happy Landings, Car Surfer and Hard Landings are well-considered to be far from competitive.. What is the drawback from using Valziprams then? Virtually NOTHING.
Not only those tanks have stronger explosions but also a bigger radius, which gets amplified by explosives 3, which more people will use now because their beloved high-burn-fuel is getting a nerf. It is also statistically impossible to run away from a well-placed brick-remote-det on these tanks with explosives 3..
P.S reply: Big cars having bigger explosions is not logical because gas tanks are gas tanks. They're still consumer-sized vehicles.