Those who asked WHERE you are talking have it right.
In Africa, Germany can spread thin. If the UK and US decide to take Africa back, there is littler Germany can do about it (without leaving Russia too strong for it to matter in the long run). So spread thin, grab IPC’s while you can, and be happy for it.
In the Siberian Russian territories, Japan can spread thin as needed to take IPC’s from Russia. Every territory Japan can take from Russia is that much less infantry for Germany to blast through. And if Japan keeps supplying cannon fodder for the slash and burn assaults on Russian territories, being spread thin just means that Russia has to attack small clusters of Jap infantry using their own infantry in less effective attacks.
In Europe, spreading thin can be fatal, unless you spread very far very fast. As an example of that, the radical German strategy of simultaneous attacks on Karelia, Eastern and Caucuses is an example of spreading thin that MIGHT be worth it. You make Russia build in Russia (even if they re-take Karelia), and prevent them from massing there again by the threat in the Caucuses; and open up some “enclosed” territories that you can hold with no forces (so long as Russia never holds Karlelia at the start of their turn).
Lastly there is the “apparent” spread thin that is really massed troops. This would include a strategy I posted elsewhere of Two Japan fleets, one in Japan, the other in Australia, that can both converge to hammer any US assault in the central pacific. You may look spread thin, but the reality is that you have forces massed for attack.
But when it comes to Karelia and Eastern Europe, there is ONLY one answer: The more, the better… ALWAYS!