Small Propellanes Stability
Fulfilling the request from @Mockingbird, there is a quote from (1, p. 90)


  Surprisingly, [1.1.1]propellane is somewhat more stable to thermal
  decomposition than the next larger propellane, [2.1.1]propellane,
  indicating a reversal in the trend of increased reactivity with
  increased strain. To understand this observation, it is important to
  recognized that the energy of both the reactant and intermediate
  influence the rate of unimolecular reactions that lead to
  decomposition. In the case of propellanes, homolytic rupture of the
  central bond is expected to be the initial step in decomposition. This
  bond rupture is very endothermic for [1.1.1]propellane. Because
  relatively less strain is released in the case of [1.1.1]propellane
  than in the [2.1.1]- and [2.2.1]-homologs, [1.1.1]propellane is
  kinetically most stable.(2)
  
  
  
  Another manifestation of the relatively small release of strain
  associated with breaking the central bond comes from MP4/6-31G∗
  calculations on the energy of the reverse ring closure.(3)
  
  
  
  The thermal decomposition of [1.1.1]propellane has been studied both
  experimentally and by computation(4)


The book chapter further discusses reactivity of [1.1.1]propellane, if OP is really interested in this area, I would suggest to grab the book in the library or focus on the papers (2-4) the book references to.

(1) Carey, F. A.; Sundberg, R. J. Advanced organic chemistry. Part A. Structure and Mechanisms, 5th ed.; Springer: New York, 2007.
(2) Wiberg, K. B. Angew. Chem. Int. Ed. Engl. 1986, 25 (4), 312–322. DOI: 10.1002/anie.198603121
(3) Adcock, W.; Binmore, G. T.; Krstic, A. R.; Walton, J. C.; Wilkie, J. J. Am. Chem. Soc. 1995, 117 (10), 2758–2766. DOI: 10.1021/ja00115a011
(4) Jarosch, O.; Walsh, R.; Szeimies, G. J. Am. Chem. Soc. 2000, 122 (35), 8490–8494. DOI: 10.1021/ja994043v