Last week I got to go to Rome for the first time and got to see all the places I've read about.
One thing I read on most of the sites that talk about Constantine's Arch is that the 8 Dacian statues represent prisoners. Well, having seen the Arch "live" this seems to me like total crap. None of the statues have their hands tied in any way, they are huge and also they dont have the facial expression of captives. They look freaking imposing! Also, they are not under, but rather right next to the inscription that praises Constantine.
Now, I also spent a lot of time studying Trajan's column, and there I could clearly see the Dacian prisoners (they all had their hands tied behind their backs, were kneeling, a bit smaller than the Romans, and looked defeated).
So my question is: do we know why the statues are really there?
Here is a picture i took: http://imgur.com/dRuuSyb
We should also always be careful not to assume what we expect from a depiction is what the Romans would. The size, for example, is perfectly ordinary in the context of monumental art (the Gauls in the Pergamon Altar, for example, are larger than life size). That being said, there are a few important items of note. Most importantly, these images were not originally carved for Constantine's Arch, but rather for a temple dedicated by Trajan. Most of the images on Constantine's Arch, in fact, are taken from earlier monuments, which is often seen as a way of appropriating the image of past images to depict himself as the restorer of the glory of Rome. Trajan is generally well known for depicting him enemies with an element of nobility and spirit, as you may have noticed on his column. So the Dacians may have simply been decorative elements and also served as a reminder of the fierceness of the enemies Trajan defeated.