The 20 round mag a friend has for his Glock is only an inch or so longer than his 17 round mag (which is flush when inserted. )
A 20 rounder would only add 3 rounds to a stock hi-cap, and its a double-stack mag, at that! Why would it be 6" longer?
The real disadvantage of the BIG magazines (33 - 50 rounds, whatever) is that they're really HEAVY!