To put it briefly, the main thing is the size of the windshield.
Because when looking at a picture of a non-street derived race-car, you have no obvious reference point to quickly decide what size it really is, so your brain grabs the biggest thing that looks normal: the windshield. And it assumes that all windshields must be somewhat as big, because you still have to fit a driver behind, right?
Well, actually, no. Street cars, and thus streat-derived cars have a much, much bigger cockpit, because a driver and 1 to 4 other persons must be able to fit in it somewhat comfortably, and have a good vision out. The passengers’ legs are more or less contained in the cockpit, for example. For comparison, here’s how the cockpit of an LMP1 car looks like:
Pretty cramped, right? Look, the driver has to be laying almost flat to fit in height, and his head is much farther back than your brain would have you think when looking at the exterior of the car. And even if by tradition LMP1 have to be able to house two human beings, realistically you’re not going to put anyone besides the driver. He fills almost all the room there is, and he takes up almost half of the total length of the car!
So, to sum it all up, here's how your brain sees an LMP1 car:
Quick and dirty photoshop of what the 919 would look like if its windshield were the size of the 911's. Yooge!
And here's how it really looks like:
Add to that the fact that we generally think that everything we see in a picture is bigger than it really is, and if you happen to see an LMP1 in real life up close, you’re up for quite the shocker!