I can say that this time (4th child) I felt BH contractions by 8 wks along. I'm absolutely certain, though few people believed me, either. I sort of know my body...very well. So I'm certain. I can't remember on round ligament pain. It wasn't long thereafter, though. I've read that BH ctx start at about 6-8 wks pregnant, so I don't see how its impossible.
How long ago was your first child? My children have all been born within 18 mos of the previous...so a lot of things happen differently with me than with most moms who go 2-3 yrs of spacing.
And yes, your uterus expands much more quickly and sticks out further more quickly (so that you feel bigger) with subsequent pregnancies. You will "stick out" more with the subsequent pregnancy the weaker your ab muscles are. So, if your abs didn't really recover completely after the last pregnancy, you'll notice this, too.
One thought though...have you had an ultrasound? Rapid expansion (leading to more sensations early on) can also be a symptom of multiples. Not to get you concerned, or anything. LOL