I had a big difference between my first two kids. My first was a big nurser, I had 19 mos w/o periods and then another 12 mos or so w/o ovulating. Then I was pregnant with #2. Tandem nursed and periods returned at 9mos. Things I think which impacted the quicker return:
#2 had very different sleep patterns, she slept longer at night
less comfort sucking, I think I had so much milk with the tandem, it wasn't as comfortable for her
Very different nursing personality, so less sucking in all
I was still a "healthy" weight, but was VERY thin after my first for a while.
Another thing I wonder about is milk ejection reflex and also "over" supply. If the baby is getting flooded reguarly, there's probably less sucking overall.
I remember two women talked about NICU experiences and a quick return to cycling. I wonder also if early separation can have and an impact as well, even if nursing then goes w/o a hitch.
Amy