Each resource hex generates income based on its type, cmall cities do it with 0,25 mpp per str, other with higher, capitals even do it with 2 per str, other with different numbers, mines are the best with 3 mpp per str. So your income is heavily based on the str of the hexes, where barely supported hexes quickly drop to 5 or even less, and with fightes, it can lower until repaired, with one per turn str gain.
Also most major has an economic modifier, that is increased with the economy tech, while most minor has a modifier of 1, so they generate the base number, russia for example has a low (30% or something like that) base modifier, so they only generate 3 mpp for every 10 on the map mpp generation, but the econ tech boost them the most, since it increase this number with 15% (AFAIK), so they effectively get more than germany with their high base modifier. And also this is why romania is so great for russia, they have a base of 100% or 1, so their stuff produce at full.
Now for the convoys:
If a country has no convoy link to its master, it generates normally, but with maybe reduced str. But if there is a convoy, that convoy already represents some of its economy. so for sweden, that has a german convoy, germany gets some of its income with the convoy, and if sweden become a german minor, germany gets only what left after the convoy, so in this case, getting sweden or norway or finland into the german camp is much less beneficial, since the economic gain is much less (maybe even zero), and their forces are small at best. Also sweden will not have a land link to germany, so most of its resource spots will work from local supply, 5 or 8, so they will generate only a small amount, compared to a landlinked country, like belgium or netherlands.