Ore gen is different per BoP biome, as far as I know. That is, certain ores are rarer/more common depending on which biome you're in.
I know we have also found some very large iron deposits under our Lush Swamp. Iron may be more common across the board.
For what it's worth, if iron is more common and the veins are larger, it increases the chance that you will find 2 veins touching each other, leading you to believe the vein size is massive. We have found many 10-20 block veins and a few 'super' veins that are clearly multiple merged veins. I think our best was around 32 in one vein.
A good giveaway that you found multiple veins is an exceptionally large deposit that is only touching at one face near the middle. That indicates two veins that just happen to touch on the edges of each. It's not always the case, but our largest hauls are usually like that.