I just always decline expedition offers I rather the AI use and supply its own terribly designed divisions while I use my own armies with properly designed divisions for my own needs. Never felt the need for all that extra manpower anyway, even when playing minors like the Dutch or Denmark(and going Communist at that for the achievement so no extra manpower from Militarism and Military Youth). Sure, your expectations need to be adjusted, don't expect to conquer the entire world on your own with Communist Denmark, but with the Dutch I basically did that by going Fascist and splitting it with Germany. In fact I have a save in 1950 now with me as the Dutch controlling a small part of France, basically extending my borders just a bit, and of course Belgium and Luxembourg(mostly for the achievement but also why not) as well as the entire UK, entire USA, most of Canada, the entire British Raj and a bunch more territories in South East Asia, Central America from Mexico to Panama barring the little islands as well as most of the Middle East(barring the coastal nations which Germany and Italy made into Fascist puppets(Egypt, Israel, Syria, etc). All that with the Dutch's tiny manpower and no expeditions(though I did once accept a 92!!! Italian expedition accidentally but never really used them much just wanted to get rid of them until eventually I discovered that disbanding them will do just that, return them to Italy rather than get rid of the altogether), though I did go to all adults serve since I was indeed getting low on manpower(only a 20k or so left before switching from extended conscription).
Speaking of that game, if anyone wants a true world conquest with the above scenario leaving the Axis than betraying Germany and basically fighting the rest of the world barring neutrals in South America and the like than let me know. All techs including 1950s Jet Fighters and Tactical Bombers(working on the strategics now, never really used them so didn't bother researching the line earlier) available, and probably for the AI too, tons of resources but also plenty of shortages that you won't be able to get from Germany(as I am now) since you're betraying them though you have more than enough industry to sustain you Imho. Oh yeah and 48 killer armored infantry divisions with super heavy tanks, AT, SP and AA alongside a bunch of infantry(for organization) and rocket arty. These divisions wreck hard, though I was surprised the 1.1 AI manages to pierce through these massive 40 width divisions pretty regularly despite using 1 super heavy TD, 1 super heavy tank, 1 super heavy AA and 2 or 3 super heavy SP(forgot how many) all with a bunch of armor upgrades on top of being super heavy(had tons of land experience to spend) along with the usual 9 or so infantry and some rocket arties to fill width to 40, oh and with 5 support companies too(the scaling ones like engineers, recon, maintenance, etc). If anyone wants to play out this apocalyptic war than I can send you the save file.