My memory of XW:A is a little fuzzy, but may I suggest something:
the marauder corvette was used mostly by pirates, smugglers etc. in XW:A. Whether it's a coincidence or the developers specifically used that name for the ship, but it sounds like a pirate vessel. I would suggest that those two ridiculously over-sized barrels are actually massive turbolasers (or ion cannons) that are generally far beyond the capacity of the ship's reactor to power them up for combat use, so they're more like one-time affair. If it's a pirate vessel it would be used to attack convoys or loner merchant ships (something that can fit Rebel mission profiles, too) then those huge turbolasers may be of use - a single powerful shot to cripple any freighter that refuses to surrender in the shortest amount of time.
After firing, they small ship reactor would need some time before charging them for another use. It's an overpowered hit-and-run weapon, not a straight "broadside exchange" capital ship weapon.
Okay, I pulled this off mostly from my imagination, pirate lore and memories from XW:A, but IMHO it makes more sense than having a 190m slow corvette with weak shields and a small reactor being a purpose-built anti-capital ship... or anti-fighter, it just doesn't look right for that role.