Well the definition of a LMG is pretty broad, what we would call Assault Rifles, such as the Steyr AUG are actually defined as LMG's. As a gamer when I say LMG, I actually mean ones that can also be deployed as a Medium Machine Gun. These are typically belt fed (although not a requirement), bipod/tripod mounted weapons used for sustained fire.
So which LMG are you talking about? As I look across the typical LMG's suggested, such as the M60, M249, Mk48, etc they all use 5.56x45mm or 7.62x51mm rounds. This means that although the bullet is the same size as the assault rifle rounds, the casing is longer and therefore packed with more powder. This means that the muzzle velocity of the bullet is a good 100m/s faster than an assault rifle.
The SCAR-L can fire both of these rounds, so that means these weapons would have at least the same damage per bullet as the SCAR would. Also the ROF for these LMG's is pretty comparable to the ROF of the assault rifles, if anything they generally have a higher ROF.
So there you are, you have a SCAR with a huge clip, low recoil, excellent accuracy, and a good rate of fire. Remind me how that wouldn't be overpowered again?
Gibbys Bren Gun suggestion, as he has stated above, took these factors into consideration and came up with an acceptable list of drawbacks to counter what would otherwise be a weapon that rapes everything. It has these powerful bullets sure, but its rate of fire is slower and is awkward to use for 1 man.
Oh and FYI the Bren gun can support a 20 round L1A1 SLR magazine, a 30-round detachable box magazine, and a 100-round detachable pan magazine. I'm sure I read there was a 50 round mag available somewhere, but I can't remember where I found it.