One good indication of how large trolls are meant to be comes from Tolkien's own illustration of the three trolls in The Hobbit. This picture, in the chapter "Roast Mutton", shows the trolls looking out from the trees at an approaching dwarf. Judging from the size of the barrel in the middle ground, they are enormous - over twice the size of a man. Even if the barrel was a small keg, they are still about the size of the creatures depicted by PJ in the films

As for orc/goblin politics - they existed in the period in which The Hobbit is set and continued to exist later, when Sauron (and Saruman) had overlordship over them. The orcs of Isenguard clearly saw Saruman as their chieftain and the orcs of Mordor saw Sauron the same way. But the orcs of Moria had greater independance - pursuing the Fellowship downriver to kill them in revenge for the death of their chief as the Company was escaping from Kazad-dum. So it's likely the orcs outside of the direct control of Mordor or Isengard maintained their own leaders and tribal structures.

Cheers,
Thiu

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