To put some numbers into perspective, Serbs + Monte + Albania together had some 400-450k troops in 1914 and Austria had about the same. (Considering only 5th Army). i.e. Historically. [The Austrian 5th and 6th armies in the Balkans together had troops equivalent to 1 army]. Of course these are raw numbers and the sword tip (the numbers you get in game) is usually about 30% (assuming teeth to tail ratio of 1:2+). Clerks, typists, messengers, administrators, cooks, cleaners, workers, nurses, baggage trains/logistics etc. is all abstracted.
So, 150k means roughly 4/5 corps, this is close to what you start as Austria, you get 2 additional corps due to 2nd army + its HQ and then rail in 2 detachments or something, thus making it about a million troops (slightly lesser) in the area of your own, add Germans and it balloons.
If you send 2nd Army into Serbia that doubles, add Germans and it is like throwing over 1.2 million men. That makes it a 2:1 or even 3:1 superiority and thus the fall of Serbia or Monte shouldn't be surprising.
Only issue is Albania should mobilise faster, Greece should provide some allied supply and either the French or the BEF should be positioned into Balkans if you want to save them. Maybe even pull troops from that detachment in Cyprus or even Egypt etc.