How many german divisions do you reckon could have been deployed in Greece? They have the single rail line to support said deployment.
Not an expert but the numbers I had seen were a German infantry division needing 250 tons of supply per day and a panzer division twice as much? Single railroad to the south of Thessaloniki with one line connecting it with the Bulgarian rail network and one with Serbia. So in theory no more than a dozen divisions to the south of the Olympus, more as you go north of it?
Now start subtracting from this losses to the carrying capacity from sabotage and allied air attack, troops on occupation duty and troops fighting partisans, that Lufwaffe units in Greece need to be supplied from the same railroads then take into account that the same rail line needs to keep in supply every single German to the south of Belgrade....
This of course does not take into account whether the divisions will be available in the first place while Germany will need to be rushing divisions everywhere from Italy to Russia. The obvious play on the German part would be to secure Bulgaria at all costs on their side. If that means handling over all of northern Greece Thessaloniki included to the Bulgarians... well it doesn't cost the Germans anything to do so.