A vehicle with a single occupant can take one action per turn in addition to moving. If they want more than one action, they have to take a -1 per additional action to each action they do that turn. So, a person taking 3 actions will take a -2 to every action they make that turn - including defense rolls.
So yes, you can load your vehicle down with twenty light railguns, but you can only fire one per turn, at a single target[1] unless you want to get hit a lot and miss a lot, AND it will cost a lot - enough for the other side to purchase several railgun-toting vehicles. Quite a bit different from Battletech where you can fire all your weapons at different targets each turn without any penalty other than heat. Each of the weapons are sufficiently lethal that multiple weapon firings are not needed. If you're curious I can provide examples.
The point behind the size chart is to provide guidelines for what can reasonably fit inside a given vehicle based on its size. For example, you wouldn't put a Heavy Rail Gun (size 12, generally mounted in tanks) in an APC (size 8-10) or a Heavy Gear (size 6-7). You might find one in a Strider (size 10) with one in it, but it will take the stabilizer mount perk for that weapon, and will have to stop moving to use it. It also won't have a lot of ammunition for that weapon.
[1] There are, of course, rules for walking fire and saturating fire with high ROF weapons. There are also weapon links, but they get expensive fast, and you can still only fire at a single target with them...