I see what you are getting at.
I took some measurements. The Losi 8mm driver has a hollow section that is about 26.3mm deep before it bottoms out at the solid shaft. However, the hex driver part of it is only about 5.5mm deep.
So if you are trying to drive a long screw it can only be inserted into the driver 5.5mm using the 8mm driver. The smaller the driver size, the less a screw can be inserted. The hex driver part of the 4.5mm is only about 3.5mm.
Hope this helps.