Both sizes have changeable ram. The difference is the 21.5's ram is under the logic board etc etc therefore not really user replaceable. The 27's ram is user replaceable.
The HDD I kind of understand though not being able to add more RAM.. Well that's Apple for you.
I'm not sure about anyone else but I've have changed both RAM and HDD in iMacs and Macbook Pros for the longevity of the overall life of the system. Yes I am a little bit of the Apple fanboy though not being able to update either or both RAM/HDD has meant that I've put off updating to a new iMac or rMBP.
But the way it looks as though Apple is going I'm guessing you wont be able to user service too much longer...