The further they retreat before being surrounded, and the wider the level of encirclement, the more supplies should be trapped. If you have a strategic encirclement that pockets a whole Army/Army Group, then the trapped area might include a supply depot. If it includes a port then you are going to have a long long siege, as some new supplies might continue to get through. Sevastopol and Courland spring to mind.
If understand the system correctly, each province (along a supply route - i.e. the "green ones" in the latest screenshot) will have it's own stockpile, so yes the larger the area that is encircled, the more supplies would be available within the pocket.
However two things come to mind:
a) A pocket that is wider rather than deeper (relative to the front line) would have less supplies within it - because the supply lines emanate from the capital, and presuming that the capital is
usually behind rather than to the left or right of the front line. So if you encircled 2 divisions in a pocket 4 province
wide (i.e. including 2 province that are not in supply) they would not last as they would had you encircled them in a pocket 4 deep (i.e. including the provinces behind them relative to the supply line - all 4 are in supply). I'm not suggesting this is a problem, just something to figure into everyone's equations when developing plans.
b) Trapping more divisions in the same number of provinces would lead to those divisions crumbling faster - i.e. 4 divisions in 4 supplied provinces using up supplies 2x as fast as 2 divisions in 4 supplied provinces. It seems obvious, but we will have to plan accordingly; do you surround less divisions (which require less divisions to keep them surrounded) for a longer period of time or surround more divisions (requiring more divisions to keep surrounded, and being more capable of breaking out) for a shorter period of time.
Just some stuff to think about.
Hopefully they show a reasonably zoomed map screenshot of the Soviet (western) border region sometime soon, so I/we can begin planning/drooling over that most grand of campaigns.
Perhaps HQ's serve as "routers" for supply pulling by units under their command?
For example, that way the issue with supply line adjustment in the Far East would not arise - the Far Eastern Front HQ would pull all the supply towards its own province, and then it would fan out over much shorter distances to subordinate units and HQs.
I'm hoping for this as well - the units we've seen so far in screenshots look like individual divisions (I may be wrong) but I hope for corps or larger the supply route goes to the HQ, and then fans out from there to the divisions. It would make each HQ a tempting target for a rapid focused attack, as they frequently were/are in real life, as it would (at least temporarily) disrupt the logistical situation of its subordinate units. It would also make protecting your HQs a top priority. Finding and heavily bombing an enemy HQ could also disrupt the logistics (and by association the plans/actions) of a large number of units.