Formation of phalanx

[Khorak]

As long as those pikes are, when they are leveled they are not the best protection against arrows and javelins. So the first ranks needed some added protection. Not much but it all helps.
Further, the buckler was not as small as the Levy Pikemen's shield, it was in fact around 60cm in diameter, which is not large but certainly not too small either. So I don't know if I would call it a buckler.
The phalangites were not always used as pikemen, but also as swordsmen and javelineers when the terrain was not for big blocks of pikes. In such cases a shield would be a serious help.
Lastly the shield was strapped to the forearm but it also had a strap up and around the shoulder and neck (just like a shoulderbag), making certain the shield kept facing outwards as well as helping the left arm keeping the pike leveled. Without the strap the pike would become quite heavy for the left arm after a while, but with it a lot of the weight was transferred to the back and neck.

The RTW one does look a lot more weedy than 60cm....a 60cm one should easily be as long as the forearm of any man with it, and obviously extend in all directions quite significantly. I'll have to go look again now, because I reember the RTW ones being smaller than the length of a mans forearm and quite easily qualifying for the term 'buckler'.

[Uesugi Kenshin]

I think i would call just about any relatively small shield that straps to your arm, leaving your hand free a buckler. Is there any formal definition?

[Kraxis]

Well the point about this shield is that not only did it have a neckstrap (glad I could help with that one) but it had a similar lining of rope as the aspis. So in a closer melee it could be gripped with the left hand and much more maneuverable.
I can't remember perfectly how the shield was carried on the arm, but it seems like it was carried like the aspis, with a central armhold (sort of 'put-your-arm-through' thing) where you would have your elbow. Thus the hand would extend beyond the edge and be able to grip the pike.
While the aspis had the problem that it would extent a lot of its protective area to the left, the small Illyrian shield would in fact be dead center on the body (you would have to cross the left arm across the body to be able to grip the pike), so you would gain the most from it.

The shields in RTW are indeed tiny, at least in the case of the Levies. The Phlanx Pikemen seem to have more acceptable shield, as do the Silver and Bronze Shields. But I have not done any serious investigation into it.