-The reason a small guerilla may 'win' over a large power is simply (IMHO) because the success criteria are different.

The large power needs to eradicate the guerilla (preferably without too much collateral damage in the population the guerilla is hiding in, as that would -in addition to be plain wrong - increase recruitment to said guerilla.).

The guerilla simply needs to not be eradicated. As long as it exists in any meaningful sense, it will tie up vast resources for the adversary.

Or, phrased simply - the large power needs to win to win. The guerilla needs to not lose to win.