Why? The German assault was lead by paratroopers. That´s very light infantry with no tanks, next to no artillery and few SMGs. Furthermore the only tanks send to Norway were Pz.II tin cans.
German air superiority? How? In you TL the Norwegians mobilize much more quickly and that greatly decreases the chance of the Germans capturing and holding the key airfields in southern Norway. Without the airfields the invasion is DOA!
By the way, my book from 1960 says if all six Inf.Div are mobilized the size of the Army will be 110,000 men, not 300,000.
Tin can counts when the only AT weapons available to the enemy is dynamite, you know.
German Bf 110 and bombers can reach southern Norway from northern Denmark, and even if the invasion goes much less well in this scenario, I think the Germans are bound to capture at least one airfield anyway.
Yes, the line army will be around 100 000 men, and local defence units, landstorm, depots, reservers etc will be a further 200 000.