Why is the Tropylium carbocation less stable than the tricyclopropylmethyl carbocation?

Tropylium is highly stable due to conjugated system, that being resonance stabilized and the number of canonical forms of tropylium is more. But it is less stable than tricyclopropylmethyl carbocation because tricyclopropylmethyl carbocation undergoes a strong  stability mechanism factor that is sigma resonance or dancing resonance  (sigma-tropic rearrangement ).

What is Bredt's rule ?

The Bredt’s rule is relates to bridgeheads with carbon-carbon and carbon-nitrogen double bonds. This rule is  first discussed by german Scholar  Bredt’s  in 1902.

According to Bredt’s rule a bridgehead carbon atom of bicyclo compound cannot be sp2 hybridised or in other word a bridgehead carbon atom cannot be form double bond. unless the ring that contains at least eight atoms 

In Other word Bredt's rule state that planarity is not possible at bridge head carbon of bicyclo compound till one of the ring size is (8) eight member or more.

   

ILLUSTRATIVE EXAMPLE: Bredt's use for the predicting major and minor  product of elimination reactions.