Just because the cars are not that big, it doesnt mean then inpact force is not big. The cars are probably traveling at around 70-80mph or more, and the cars carry a lot of mass behind them. We see how small the cars are and think the inpact can not be that great, but its an incredibly big inpact. Add to that the fact that cast aluminum is rather brittle and you get what happen to that engine. The .12 probably dont have that happen because TCs use foam bumpers, therefore minimizing the force of the inpact. 1/8th cars dont have foam bumpers, they just have that plastic bumper that transmits the full force of the aluminum chassis behind it.
I still dont know of the engine that broke is the one on the car that got hit, or the one on the car that did the hitting.