Once again, this appears to be a thread best answered by "keep playing the game, you'll see."
I'm into the summer of 1943 in an Axis GC. What I've noticed about Soviet pockets is confirmed by the longtime playtesters:
-- They're easiest to take down in 1941, as they should be.
-- It becomes harder to form them in 1942 and 1943 as Soviet forces become better at defending en masse.
-- The larger the pocket, the longer it takes to reduce them (unless you're using your panzer forces, but then *you* are the one playing ahistorically, not the AI-- and your overall advance will suffer due to the lack of your panzers at the front).
-- The AI definitely resupplies key forces within pockets.
-- Dug in forces within pockets survive much better than those that aren't: I've been HELD any number of times when underestimating the amount of force required to cause a retreat / surrender.
Really, the game mechanic isn't perfect-- there are exceptions to every situation-- but as a generic model on this time scale, it does a fantastic job.
Also, remember that the AI, while good, isn't as good as the lamest human player. Humans would fight not to get pocketed, would fight hard to get out of pockets, would form relief forces to get into pockets (which provide instantaneous pocket-wide supply when successful!), and would launch heavy air resupply of pockets. The AI does all these things, but it just doesn't do them as well as a human player would. Can't criticize that too much.
THAT ALL SAID... if there was *one* thing I would tweak about pocket defense, it's that armored units appear to surrender far more quickly than infantry units. I can rationalize this-- tanks need fuel, far likelier to be scarce in any pocket-- but playing against the AI my repeated experience has granted me the foresight to know that when reducing pockets, if I see a lone tank brigade, division or even corps, I can generally use a fairly weak Axis unit to attack it with a likelihood of causing its surrender, reserving my stronger forces for dealing with Soviet infantry (which tend to better defend in pockets).
While I'm fine with the pocket mechanic at it is, might be worth a look at whether there is something to this, and that armor in pockets shouldn't be penalized as much (IRL, it'd be a case of fewer armor available, but that armor would still be capable as long as the gas didn't run out... maybe a higher attrition mechanism for armor in pockets, but no other penalties?)