This is a good question, because the engines during this era weighed remarkably little. There is not really a single reason, but I would say that perhaps the best blanketing reason we can attribute it to, is that they were designed right from the ground up to be as light as they possibly could. Road engines are usually far more robust in design nature.
http://sd-2.archive-host.com/membres/up ... BMW_F1.pdf
This a link you might find interesting, I believe an engineering student who posted on this forum did this as his thesis (please don't shoot me if I'm wrong) but it is quite interesting reading just how large the improvements were that BMW made year after year in terms of weight reduction, and 'packaging' envelope. In fact the P85, the final of the BMW V10 fully assembled was only 82kg!!! So we can imagine that things like casting wall thicknesses were an absolute minimum, and what ever material could be removed, was.
I think what we see is that when the V8's came in, there was a very conservative 95kg minimum limit which didn't exist during the V10 era, so the limiting factor was really only how good the design was, and perhaps the application of particular materials for particular functions or load conditions. Remember the material scope during this era was a little bit bigger, so there is something to be gained there.
Lastly, don't forget the basic geometry of these engines, there stroke is typically somewhere around 40mm. This is important because your conrod length is typically some multiple of your stroke length, so the blocks of these engines had remarkably low deck height, there is plenty to be saved there. Typically road engines are not so 'oversquare' in nature so there not really even comparable in that regard.
