I'd like to point out that the 5.6x39 is a regular old 7.62x39 case necked down to .22 for sporting purpose in russia. I think lapua made it for hunting since it had better terminal performance than the 7.62 at long range with less drop.
The 5.45x39 is a smaller diameter case (as you've seen) which was made for the same reason the 5.56x45 was made: less weight per round = more rounds a person could carry.
I've also heard tales that the 5.56x45 and the 5.45x39 both used smalled bullets with lighter loads because they were never intended to go past 200-300 yards or so. I heard this many years ago and have no evidence to back it up though.
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