I think Europe/North America/Australia/Japan/Korea market sizes are quite reasonable.
I'd like to see especially USSR/Russia and Brazil markets sizes reflect a bit more reality. In Russia/USSR unless the flight is to/from SVO/DME/LED/TAS or KBP there is no aviation market in Russia/USSR. Yet airlines have extensive networks in Siberia. Maybe make Russia similar to domestic China which has ample demand in the game. Same in Brazil. Unless the market is GRU/GIG/SDU/CGH/BSB, there seems to be little demand in rest of Brazil. Id like to see more domestic demand from SSA/REC/FOR/CNF that reflects real life so airlines could hub there. Africa is another problem. There is virtually no Africa-Africa demand except in Eqypt, Morroco, South Africa and Ethiopia.
I know this has been planned in the past. But should be too hard to bump up these market sizes.