Crossbow bolts were not thinner. What makes you think that? Bolt weights vary a lot as there were lots of variance in crossbow draw weights. 60-80+ grams appear to be common and most likely thats for the smaller hook and belt crossbows as they were most common. There are also lighter bolts like the Dale bolt used used in Sweden in later 15th century of 35-45 grams. AFAIK some bolt heads have been found that had a weight of around 70 grams, so that would be a 100-110+ gram projectile.
Generally I’m going of TV evidence as the royal Armouries at Leeds don't have much in the way of actual ammo for the missile weapons, (probably a safeguard in case someone tries nicking anything). They just look thinner on TV.

My main point of contention on this though was that a Longbow arrow is a LOT longer than a crossbow bolt and also has a chunk of metal at the front, this usually looks much bigger than the comparable pieces on Crossbow Quarrels, (at least to me, you might know a bit better). Thus, it follows that the Longbow arrow should really be heavier, (IMHO). Even if it isn’t thinner.

Knowing a bit about ballistics (not much mind, and certainly not the formula), from reading discussions on it on the web and in books i know a Heavy projectile will carry better, (in both max range and penetrating power), for the same momentum, than a lighter one.

Of course a Crossbow can and will make up for this somewhat with it's much better power. Indeed a sufficiently powerful Crossbow could well outstrip a longbow in terms of max range and penetrating power at the longbows max range. The power required would be pretty hefty to do it mind though, and really would take far too long to reload as it would have to be a hefty winch job for sure.

In effect I just don't believe any Crossbow able to match a Longbow in lethatality at that range would be a practical battlefield weapon due to the reload time being so great that a lower range, lower draw crossbow would be able to get more volleys in despite it's shorter range.

Your point about Crossbow manufacture and training is well made, but at the same time, if the Crossbow was really better than the longbow in overall killing ability, they would have switched, the very fact that they didn't tells us they felt the crossbow was inferior. Of course that would have required a switch in manufacturing centres, but so too did muskets so...

As an aside, am I the only one who thinks gunpowder weapons are too long ranged, (considering they couldn't hit the broadside of a barn at 100 yards in the American civil war, I doubt they could at the time periods represented here).

p.s. I'm not saying you ARE wrong, it just seems odd when I try and look at things in the light of logic and science, thats all.