>>64479899
>Can somebody explain why were WWII SMGs so heavy and large with magazine far forward?
I mean your pic is a Tec-9 which is technically not an SMG but a pistol but was originally intended as one and early versions were easily convertible. So how would it fair as an SMG in the war? Well for a start it weighs 3lbs, has.no stock, a short 3in barrel and given that the bolt is both light weight and has no travel distance its rate of fire is probably somewhere in the ballpark of 1000-1500rpm, so extremely close range and basically uncontrollable.
Okay, lets make this a little more viable for combat use. First lets extend barrel to 7-8in and add a stock, something not to wirey so you can stabilize the gun. Next lets tamp down that rate of fire a bit, say 700-800rpm which is middle of the road for the time. You can do that in two ways without needing a more complex bolt, more weight or longer travel and since a heavier bolt will negatively effect accuracy, lets lengthen the travel a couple of inches.
At this point the gun probably weighs about 5-6lbs, add on another 1.5lbs with a full magazine. Now this gun is going to be dragged all over Europe so flimsy construction is not your friend, maybe add another pound of steel to make sure she doesn't dent easily. Unsurprisingly the Tec-9 WW2 edition now looks a lot more like its contemporaries and weighs about 8lbs loaded. Compare to >>64479913, MP-40 is a bit longer and heavier but that is due to a 10in barrel and a little more travel, RoF is 500rpm.

>>64479920
Germans and Brits started the war issuing them to platoon leaders but that quickly became squad leaders too and Brits gaven them too assistant squad leaders as well so they had a lot of presence. In the US they were more of a truck drivers weapon but much like the BAR, squads often had way more than they were supposed too. Eventually the brass issued company weapons pools with MG's SMG's, BAR's and Bazooka's so infantry could better customize.