I'm glad you started this topic, because it's answered a lot of questions that I've had, too. Glad to hear about Warm N' Natural because it's all I've ever used and I haven't been able to figure out why I can't seem to get my stitches smaller. I guess on my next project, I'll try something else.
It's definitely true that seams make a difference! If you figure that you're quilting through four (or even 6, if it's where some seams overlap) layers of fabric, not even counting batting, that's a lot of thickness to quilt through. Not easy to make small stitches with all that going on. I'm quilting something with a LOT of seams, so right now I'm just being content with consistency and not worrying about size of stitches. It's making me want to take on a wholecloth sometime soon! :)