COD isn't always realistic - I imagine a 50cal sniper rifle (DSR and the XPR here) would be a 1HK anywhere in the body, suppressed or not. The M60 in real life is a heavier LMG but I don't know much about it in real life. I never liked using it in MW3 due to the obtrusive iron sights, thoguh I used it for a while in BO1.
The game treats LMGs as a fat kid's AR as they are a commonly misunderstood weapon category. However, the PKP Pecheneg (I know it's not in BO2 but it's in MW3) weighs around 10-12kg with a full 250 round detachable box. I'd still expect plenty of movement involving them. You could make huge alterations for each weapon in each category but I don't think COD has done that before as movement penalties and range of weapons tend to be grouped by category. You say you have to go prone to be accurate with them, yeah how often can you do this? The best you can hope for is to crouch, unless you're sniping out in the open, on the floor. It's the same for LMGs in exercising suppressive fire. There are very few vantage points where you can go prone anyway.
But yes, there are a few unrealistic observations in this game. The KSG slugs don't disappear after 16m or whetever it is in real life. They reach well over 150m and then they slow down. You weren't sacrificed any movement penalty, or much from what I remember, using the Javelin in MW3, and it's a heavy weapon at 22kg or so often fired from infantrymen sitting down.