Consider some of the following:
1) While the mass is significantly greater ( probably almost but not quite double the weight), the fact still is that there is also significantly greater distance ( MORE than double) from the front of the vehicle to the seated driver in the Ford and with that there is dramatically greater amount of metal and room for absorbing the impact.
2) The F 150, when embarrassment (I suspect) and risk of suit/ multiple suits (as evidenced by extensive public records) compelled Ford to redesign it for safety, it suddenly became VERY SAFE in frontal impact. Mind you all, this still addresses/ accounts for only a fraction of the breathtakingly high fatality rate (the fatality RATE refers to the fatalities per X number of cars…thus, the fact that the f 150 has been on the road for 20 or 40 or even 100 years has NO bearing on lessening it gravity of its dubious distinction as on of the most dangerous passenger vehicles). The F-150 is a very poorly designed vehicle from th standpoint of its suspension and propensity to ROLL-OVER. ROLL-OVER accidents account for the MAJORITY of SUV fatalities. AND, THE FACT IS THAT BMW ALSO MAKES AN SUV, AND IT IS NO WHERE AS PRONE TO ROLL OVER.
3) Given the fact that f-150’s have a propensity to roll over, Ford does not bother to create a structure that will resist collapse of the roof structure.
4) Neither 3) nor 2) apply to the mini, as it has a low fatality RATE as compared to virtually any domestic SUV/ truck and it has a very low roll-over risk as compared to SUVs or other cars in its class.

SO in summary, while the MINI would might not fare well in a head on collision with an F-150, its passengers are more likely to walk away from the impact than those of the F-150, and will fare FAR better than would an F-150 in a collision with a like sized vehicle or a stationary object. It is far less likely to roll over and it is far more likely to preserve the life of the occupants in the unlikely event that it does flip. All of this is INDISPUTABLE. How many cars of one is sold versus the other is IRRELEVANT as is that the two are totally different.