So after playing a few games, it seems sending volunteers is really unbalanced. A good example is the Sino-Japanese War where the Soviet Union sends say 6-7 either 40 width divisions or armor divisions. It seems many organized MP games specifically limit who and how many divisions can be volunteered to contain these issues. There needs to be more restrictions on sending volunteers especially around size/type of divisions and who/when they can be sent.
Here is a SP example of Soviet Union sending 6 40-width divisions and absolutely crushing Japan (and yes Japan AI currently does poorly against China even without volunteers but this is just nuts):
Here is a SP example of Soviet Union sending 6 40-width divisions and absolutely crushing Japan (and yes Japan AI currently does poorly against China even without volunteers but this is just nuts):