At 8 wks, chances are improving daily that you will deliver twins. I forget the stats, but it is likely over 80%likely you will take home two babies now by 8wks.
My twin B was smaller from the start as well. It can be related to one egg fertilizing a day or two later than the other, or just having a bit smaller sac on one.
THat 7 years difference is not a bad thing for your uterus--it hopefully has had a nice long time to heal, so fingers crossed. Lots of mamas have had a few sections, and gone on to have twins. It may mean bed rest a little earlier for you, but maybe not!