First off, just from looking at safety studies you find that death rates for SUV's and trucks is really high. Same goes for sports cars. Part of this is that people in these vehicles tend to to be hella crazy and speedy.
It depends on the type of crash or impact, yet most people will swear that larger vehicles are safer, hands down.
Consider crashing into something massive, such as a tree or wall or a semi truck. The object you're hitting is quite unlikely to budge, leaving it to the frame of your car to absorb the full impact. In this situation, the larger your vehicle, the more force your frame will have to dissipate. Which vehicle would you rather be in? (note: since 2004, the F150 no longer implodes on itself like that. Silverados do, a lot SUVs still do)
Now, if you're crashing into something with more give, then the more massive object begins to benefit.