1. The logistical capabilities to feed your forces from home territory didn't exist 100 miles past the border in Europe either. Bulk shipment of food was only, marginally, possible with dedicated ships. Oceanic transport of bulk grain was possible, but pretty challenging, there are exceedingly few examples of this being attempted as it was exceedingly expensive. Riverine transport was not as efficient at moving bulk calories, it was used to supplement foraging particularly sieges, but again the cost was an issue. Wagon supply was used on only a handful of sieges and was utterly impossible on the scale of many EU province sizes. Armies, more than 95% of the time, were supplied from the front. You might be sending out bands of roving soldiers to "procure" grain and cattle or you might have sutlers travelling in front of the army buying supplies ... but nobody in Marlborough's army was eating British or even Dutch victuals when he marched to Austria. Places in the new world that had plenty of agriculture (e.g. the corn belt in Mexico) could easily supply an army larger than anything in Scotland.
2. Pretty consistently, historical empires could field about 1/10th of their total forces overseas. There were transportation costs, but once in the field, these troops were often cheaper to maintain than their continental counterparts (e.g. colonial armies typically required far less powder). Army sizes in EUIV are pretty terrible across the board. Small states often could field several multiples of their current force limits in defense of home territory while the bottomless manpower pools of the majors were rarely that deep. It seems odd to be highly concerned about troop counts in secondary theaters when the primary ones are not too representative.
3. As always, the real problem is that rivals and opportunistic foes provide no real balance in game. AIs feel free to send huge percentages of their forces overseas. Truces are close to inviolate. Lack of CBs is also AE prohibitive. Neither the AI nor the player routinely faces the threat of invasion and dismemberment if they send off the whole army. Worse, even when miracles happen, the peace mechanic make it impossible for the player, let alone the AI, to force a quick concession before the army can brought home. AIs can, and do march from Korea back to Poland before you can force the Russians to give up real territory. Navies can sail from Japan to Great Britain before you can free Ireland. When everything is forced to be slow, you lose a lot of the real historical impetus for limited deployment.
So no, everyone should be able to field armies everywhere at very close to the same cost. They should just be scared to do so because leaving the home territory unprotected should provoke quick and deadly wars.