Burning DVD .iso that is "too big"

I was a bit puzzled this morning when I tried to burn a 4.37Gb DVD .iso image. Nero 6 complained that the regular DVDR disc is not big enough to handle the .iso. So I have to resrot to the plan B by first expanding the .iso into all the content files (using Winrar), and then create a new DVD Video in Nero with files I just expanded. All seems fine after then.

Problem solved, but I still wonder why it failed in the first place.