I don't care if you're a 260 lbs linebacker...ligher recoil will always equate to faster follow up shots.
^^ This.
The recoil impulse of 7.62x39mm will lift the muzzle further off target than the recoil impulse of 5.45x39mm. To fire the second shot, you have to put the muzzle back on target, and it takes longer to do that with 7.62 than 5.45. This is true even if you can bench press 400+ pounds. Now, a 260 lb shooter will recover from 7.62x39mm recoil faster than a 120 lb shooter will, but
both of them will be faster with 5.45. And regardless of who is shooting, 100 rounds of 5.45x39mm weighs the same as ~59 rounds of 7.62x39mm.
Follow up shots with a 7.62? Maybe on paper, but otherwise it's probably not necessary unless you're a horrible shot.
7.62x39mm, like 5.45x39mm and 5.56x45mm, is an intermediate round, not a full power rifle round. It was designed to exactly split the difference between the 7.62x54mm rifle round used in the Mosin-Nagant and the 7.62x25mm pistol round used in the Tokarev pistol and PPSh subgun. x39 is
not in the same class as 7.62x51mm, AKA 7.62mm NATO or .308 Winchester.
Shooting typical Russian ammunition, 7.62x39mm is no more effective than 5.45 or 5.56, and some load choices (e.g. M43 ball) are considerably *less* effective than the most common 5.45/5.56 loads. Shooting heavy softpoints at deer, 7.62x39mm has an edge over 5.45/5.56, but it's a lot more even outside of a hunting context.
There's also the fact that unless you are Chuck Norris, if you are shooting under stress or at a moving target, you can miss and have to make a followup shot.