Based on observations, it is sufficient to say that this is a valid explanation.
Subject observation: brandenburg to prussia.
Game conditions: Not ironman (never). Obviously protestant, for a simple run.
Challenge conditions: None. Even if:
- A turbo portugal that colonised 75% of the available lands, yet somehow not participated in the league.
- A megalo-gigantic denmark, that with obvious luck integrated norway and sweden, then formed scandinavia, annihilated muscovy, so no russian support.
- An ultra lucky castille, of course, as always, got the aragon, turned spain, single-handedly destroyed western north africa.
- An out-of-the-mind and a regular problem of the game, the burgundian inheritance, and suddenly the austrian forces jumps from 40k to 150k by 1520.
- But the super madness of the austrian hegemony is actually advantageous, which means they were still rivaled by the ottos. Yes, ottos were already 100k by 1500, which makes them having no rivals, and would go beyond 300k within a century.
- [At this point, there is an extreme risk on the french front. If austria goes into frenzy, then an austrian france will happen before the league, and that can field 250k by 1520, excluding its allies. Prussia with PU or vassal ansbach and bayreuth cannot cope with that, since the ottos will be always in debt.]
- A fascinating poland-lithuania, that consistently runs from scandinavia to crimea to the urals with 100k forces since 1460, and yet still rivals teutonic order (19k). Of course, they are best friends with austria and the pope.
- All other powerful protestant tags are on the austrian-papal side, a natural problem of the rivalry mechanic. The player cannot be elected as the emperor after the league war is won, since the catholic electors will be very angry, and the first election right after the war is still based on the previous elector opinions.
- An abhorent economy. It is prussia. Hyper-powerful space marines, but a single army, as there is no sufficient economy to maintain more. Whichever target that army marches into, it is a stackwipe, but the world is still too big to run over with a single army.
- A non-existing navy. It is prussia. It is pure luck if the player can beat the english navy with prussia. Ottos and russians can try to challenge them (numbers); france, spain, portugal may have a chance against them (early tradition by exploration); only netherlands can pose a threat to them (heavy ship combat ability); and without maritime ideas, prussian navy is ducklings against battlestar brittanica. But prussia does not need maritime, nor a navy, luckily.
- A miraculously bad series of rulers. Of course, it is player bias, ruler stats are generated according to etc... Anyways, never had a ruler with a diplo stat over 3. Even with prussia, frequently got 4 mil. Disinherited 2-0-4 for a whooping 0-1-4. Even Lübeck laughes at the player with a 6-6-6 god syndic. Whatever.
Subject report: Whatever. It is prussia. No challenge exists. The player reached the victory condition by 1557; after the league war, elected as emperor (the first emperor was munich, as aforementioned; fortunately its ruler was 67 years old, but of course it lived until well into 78), cleared up imperial business, always at least 4 diplo over the limit until erbkaisertum. Never revoke the privilegia; it is an unnecessary overkill. Never enact ewiger landfriede. That austria and bohemia have to be obliterated (then throw out as vassals, or however the need is be). It was pointless to play after 1580, but often checked from time to time for 1:5 ratio stackwipes. After the OP's question, continued to play to test if/when the vassals add the provinces to hre.
Case report: Vassals add provinces into hre, only when they are sufficiently small in terms of development, as the explanation given by @EquilinT.
Example 1: Resurrection of OPM hungary.
Hungary added all provinces into hre. Its development reached 224.
Example 2: Cilli being arrogant.
Cilli was given southern austria. After conquests from otto, it did not add the new croatian lands, as it had 300 development.
Example 3: Welcome back muscovy.
Muscovy was rescued from scandinavia. But it did not add any provinces after reconquest of its lands, since it reached over 300 development with reconquest.
Example 4: Polotsk the grateful.
After the same reconquest war, polotsk immediately added its provinces in to hre, since it had 57 total development.
Edit: Corrected the image uploads.
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