The mass momentun of the prop is enough to rip the front of the model . I think they choose a small prop with this thing in mind , all we know a bigger prop performs better but they also know the loads it take . Look at this:

http://www.rotax-owner.com/si_tb_inf...nfo/11ul91.pdf
The document show how to calculate the propeller mass moment of inertia for not to cross the limits. A prop when stationary spun does not put any side loads but when the disc is fliped ,gyroscopic forces act , the heavier and the quickier it turns ,the worse is the effect .

Saludos y buena suerte en la busqueda.