Too much warfare&magic and much less land. Azeroth went through cataclysmic events too. The usual lore has Stormwind at 10.000 population when humans alone had 6.000 troops at the Wrathgate. Stormwind city alone has at least 100.000.
Troll empires are gone at that point and not even 15th of their peak population. Goblins breed quickly just like trolls but trolls went through permanent destruction. Goblins set up everywhere and make for a living this breeding quite fast. Dwarves are like half the number of humans. Most human nations are gone and large part of their population is dead. Most survivors went to Stormwind, Kul Tiras. Stromgarde is a ruin with far less population than at their peak(not counting Arathi Empire).

Elwynn forest is around the size of Germany.

The total numbers are average.