I think it has to do with the length of the tube. XL size usually is built for (surprise) XL length cartomizers (45mm+), while standard cartomizer is around 35mm. Mostly preference.
I think the OP was looking for help to decide which Pro V
tank to order, the standard size or what Vaporfexion calls a regular (mini) that takes 35mm cartomizers such as Boge standards. Or, the XL which uses 45mm cartos, or Smoktech DCT, Resurrectors, etc.
As I said, the pic the OP displayed in his post looks like a regular (mini). The pic of my tank shows an XL Pro-v.