Closest i can think of is probably some of the guns used by Starcraft series Ghost units. And that´s still not even remotely close.
Hmm, and now i can´t even find the pic i´m thinking of, because the common weapon they´re seen with is smaller and more realistic, and even the one i was thinking of is probably not even half the size of your pic.
My first suggestion is that you consider the picture representation to be oversized, probably as an emphasis thing, as that kind of sized weapon, you might see it on a tank or a ship, but even if you halve the size it´s still beyond absurdly large. And even if you do that, you can still start off by calling it just that, "absurdly large".
If not, the user is definitely not limited to anything resembling human strength.
Like, if I had to guess, behind the grip and trigger would be a stock
If you look closely at the picture, those are the parts that end behind her back. At least one part each of what is seen behind her shoulder and ass are the rear ends of the upper and lower part of the "gun" handle.
Hmm... An H-shaped frame with the trigger and grip on the horizontal line of the "H" and the barrel mounted between the forward prongs.
Insufficient, but at least hopefully understandable and a start.